Valid If Problem

I need a valid if expression that checks if an input to a question exists in a column in a different sheet.

I am sure it is an easy expression that I’m just missing.

If there is no direct relationship between the AppSheet tables, then you can likely use a LOOKUP() function and check that the result ISNOTBLANK(). So:

ISNOTBLANK(
      LOOKUP([Value to Lookup], "Lookup Table", "Match Value",  "Value to Return")
)