Dynamically hide/unhide error messages

Adding to what John said above, you can show the error message in a richtextfield based on the value(s) of your dropdown. Let's say your dropdown value is getting saved in a variable local!drop. And when this value is getting populated with "A", you need to show the error message. You can refer the below pseudo-code.

with(
if(
local!drop="A",
a!richTextDisplayField(
labelPosition: "COLLAPSED",
value: {
a!richTextItem(
text: "Your error message",
style: "HEADER_SMALL"
)
          }
          ),
          {}
)
)

I have used with() here so that this expression will evaluate with each change in the dropdown value.
Same can be done with a couple of dropdown values by adding all/multiple the conditions using logical functions like "or()" or "and()".
Like - if( and (local!drop1="A",local!drop="B"),"show error message in a rich text", {})

Adding to what John said above, you can show the error message in a richtextfield based on the value(s) of your dropdown. Let's say your dropdown value is getting saved in a variable local!drop. And when this value is getting populated with "A", you need to show the error message. You can refer the below pseudo-code.

with(
if(
local!drop="A",
a!richTextDisplayField(
labelPosition: "COLLAPSED",
value: {
a!richTextItem(
text: "Your error message",
style: "HEADER_SMALL"
)
          }
          ),
          {}
)
)

I have used with() here so that this expression will evaluate with each change in the dropdown value.
Same can be done with a couple of dropdown values by adding all/multiple the conditions using logical functions like "or()" or "and()".
Like - if( and (local!drop1="A",local!drop="B"),"show error message in a rich text", {})