saving dropdown

Replace the value parameter of the second dropdown with this and see what happens:

value: property(ri!selectedTask,"locnId",null),

In general what you have above is forcing the second dropdown to not show its value when local!pullZone is blank, however nothing from either dropdown that you've shown causes any value to be saved into local!pullzone.  Maybe the if() statement should have been checking for the nullness of local!pullZoneValues instead, but I can't tell for sure based only on what you've provided.

I tried implementing the property syntax. There is no change though. I've modified the question bit more, can you please look at it and let me know.

1. Did you understand and implement the fix for the main issue I pointed out above?
2. If so, is it still not working?
3. Can you post (in a new comment) the current state of your code?

This is the error I get when i use property 

That error isn't due to property - it would be the same if you switched back from "property" to "index" (they both work the same way); the reason you're seeing this error now is because, now that the if() statement is removed from the value (which was preventing the value from showing anything at all before), the dropdown is attempting to resolve the value "00009057" against the choice values, which as the error message states, does not contain that choice.

If you edit the value of ri!selectedTask and clear out the value of "locnId", the error will probably go away.

I cleared and edited like you said and tried to select the values from pull zone again. when I select the pull zone and pick a location ID. You can see that the value is stored in rule input for the location ID. But in the column field it disappears. 

I notice you've re-added the if() statement here for some reason -- so we're back to the original problem that your "isnull()" statement is checking the value of local!pullZone, which (at least in your earlier code example) never gets assigned a value.  I think you may have meant to put local!pullZoneValues in that isnull(), because otherwise the second dropdown will never display any value.

If I just use index. on the very fist user interaction, im able to select the pull zone and location ID but on the next instance if i try to select a different pull zone, I started to get an error. In order to avoid that error I used the if condition.

So that's a different issue altogether - to handle that, you need to add a new save into the saveInto of the first dropdown, to save a blank value into the value of the second dropdown anytime it's changed.  This is a fairly standard requirement when developing cascading dropdowns where the selections in one depend on the value chosen in a previous one.

if(
      ri!taskCode = cons!IC_TEXT_TASKCODE_RESERVE_NO_FOREWARD,
      {
        a!columnsLayout(
          columns: {
            a!columnLayout(
              contents: {
                a!dropdownField(
                  label: "Pull Zone",
                  value: local!pullZoneValues,
                  choiceLabels: local!pullZoneList,
                  choiceValues: local!pullZoneList,
                  placeholderLabel: "Select a Value",
                  saveInto: {
                  a!save(local!pullZoneValues,save!value),
                  a!save(
                    local!getLocnId,
                    rule!IC_QRY_getLocnidFromMSTLocationHeader(local!pullZoneValues)
                  ) }
                    
                )
              }
            ),
            a!columnLayout(
              contents: {
                a!dropdownField(
                  label: "Location ID",
                  value:index(ri!selectedTask,"locnId",null),     
                  choiceLabels: union(local!getLocnId,local!getLocnId),
                  choiceValues:union(local!getLocnId,local!getLocnId),
                  disabled: isnull(local!pullZoneValues),
                  placeholderLabel : "Select a Value",
                  saveInto:a!save(
                    ri!selectedTask.locnId,
                    save!value
                  )
                 
                 
                )
              }
            )
          }
        )

      },
      {}
    )

This is the current status of the code. It would be really grateful if you modify saveinto and show on the first drop down

Try this.  BTW i think for the "value" of the second dropdown we can just directly access the ri property, as it likely won't have the issue of the property not existing (rule inputs are typecast to their CDT type, so the property will exist whether it's null or not).

if(
  ri!taskCode = cons!IC_TEXT_TASKCODE_RESERVE_NO_FOREWARD,
  {
    a!columnsLayout(
      columns: {
        a!columnLayout(
          contents: {
            a!dropdownField(
              label: "Pull Zone",
              value: local!pullZoneValues,
              choiceLabels: local!pullZoneList,
              choiceValues: local!pullZoneList,
              placeholderLabel: "Select a Value",
              saveInto: {
                a!save(local!pullZoneValues, save!value),
                a!save(
                  local!getLocnId,
                  rule!IC_QRY_getLocnidFromMSTLocationHeader(local!pullZoneValues)
                ),
                a!save(
                  ri!selectedTask.locnId,
                  null()
                )
              }
            )
          }
        ),
        a!columnLayout(
          contents: {
            a!dropdownField(
              label: "Location ID",
              value: /* property(ri!selectedTask, "locnId", null), */ ri!selectedTask.locnId,
              choiceLabels: union(local!getLocnId, local!getLocnId),
              choiceValues: union(local!getLocnId, local!getLocnId),
              disabled: isnull(local!pullZoneValues),
              placeholderLabel : "Select a Value",
              saveInto:a!save(
                ri!selectedTask.locnId,
                save!value
              )
            )
          }
        )
      }
    )
  },
  {}
)

(but, the important change I made is this:

)

it threw an error again

it threw an error again

What is the data type of ri!selectedTask?  If it's not type cast to a CDT, then you will probably need to change the "value" line back to how you originally had it (the commented out portion of the code I last posted).