FMS to get Last Step

Dear Experts,

I try to create fms in a UDF  to get the description of the last step in the OPR1 table.

My sql query for the fms:

Select T2.Descript FROM OPR1 T0

INNER JOIN OOST T2 ON T2.[Num] = T0.[Step_Id]

INNER JOIN OOPR T1 ON T0.OpprId = $[OOPR.OpprId]

WHERE T0.Step_Id = $[OOPR.StepLast]

But it is not working.

Thank you in advance.