A misleading design alert can sometimes appear when there is a problem with the SQL table population statement used to pre-populate a roster. This occurs in the most recent versions of the Surveybe Freedom Designer where there's more validation on the SQL statement than in previous versions. The full design alert reads:
The SQL Data Select for the Table contains a data type mismatch. Please check question types.
The design alert is intended to identify data type mismatches in the Designer, reducing the number of runtime errors you'll experience from SQL mistakes in the questionnaire. This message should appear as intended if there is a mismatch between the data type defined for the reference table and for the question that's being populated. It also appears if the question variable names you are trying to populate don't exist, which can be confusing.

To prevent the message appearing, first create the table, then the roster and questions you want to pre-populate, then add your SQL population statement to the table.