You cannot use the fields & and & together

What causes this issue?

Within the calculation formula, you referenced two fields from parallel
tables of the logical database. As a result, it is impossible to find a
suitable event where the formula can be executed.
In general, you are not allowed to use fields from parallel tables of
the logical database within a calculation formula.

System Response

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

How to fix this error?

Change your calculation formula so that you use only fields that lie on
the same path within the logical database.

Error message extract from SAP system. Copyright SAP SE.