This message alerts you what is probably a coding error:
IBM2409I E RETURN statement without an expression is invalid inside a
subprocedure that specified the RETURNS attribute.
The compiler issues this message when it finds a RETURN; statement inside a function (i.e. inside a PROCEDURE that has the RETURNS options). If this statement were executed, then the caller of the function would, if it used the result of the function, use an uninitialized value, and that could have unpredictable and arbitrarily bad consequences.
Code that produces this message should be corrected.