why I am getting error while adding data store entity in Expression Rule?

Cross validate the data with the structure of the entity. Sometimes the data queried could not be as per the data type or they might overflow with the length set.

Its fine. You can still proceed. To solve this error, just add sort info on the primary key of the column and it should be gone. I have faced it multiple times but couldn't understand the reason. 

but it's not allowing me to add sorting as long as I select constant it give me error 

Then you can avoid using the builder and directly write the code the expression. 

I tried that too getting same error 

Did you check the tomcat log file for any details?

Can you paste your code snippet?

a!queryEntity(
  entity: cons!DATA_STORE_CONSTANT,
  query: a!query(
    logicalExpression: a!queryLogicalExpression(
      operator: "AND",
      filters: {
        a!queryFilter(
          field: "coveragesiteid",
          operator: "=",
          value: ""
          /*with and without value giving same error*/
        )
      },
      ignoreFiltersWithEmptyValues: true
    ),
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: 2000,
      sort: a!sortInfo(
        field: "siteid_Pk",
        ascending: true
      )
    )
  )
)