Form routine & could not be found in program &

What causes this issue?

The form routine assigned to the function/operation could not be found.

How to fix this error?

To avoid syntax errors, create the form routine in your payroll program
before activating the function/operation.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

You can use the include PCBURZxx0 (with xx ISO code for the relevant
country or 99 for RPCALCX0) for customer functions or operations for
payroll. You can also leave existing form routines in RPCBURZ0.
Use the include RPTMOZ00 for customer-specific Time Management functions
or operations.

Error message extract from SAP system. Copyright SAP SE.