TREATAS with variables as inputs

Anonymous can you share the pbix sample data, or replace the code with the following and check if it works for your specific case.

VAR A_TreatAs =
    TREATAS (
        SUMMARIZE(P_B, P_B[ProductKey], P_B[Name], P_B[Brand]),
        ADDCOLUMNS(
            SUMMARIZE(P_A, P_A[ProductKey], P_A[Name], P_A[Brand]),
            "DummyColumn", 1
        )
    )

VAR Result =
    NATURALLEFTOUTERJOIN ( P_A, A_TreatAs )

RETURN Result

Note: A "DummyColumn" will be added to the SUMMARIZE result of P_A to make it a fully qualified table, which can then be used in the TREATAS function. This should bypass the "Function TREATAS expects a fully qualified column reference" error.