Combining field control & with online system reaction & not recommended

What causes this issue?

You have entered a combination of field control and on-line system
responses that are either not permitted or appropriate.

System Response

The system does not allow this entry, or it issues a warning.

How to fix this error?

Correct your entry.

How can this happen?

You can set the field control to ' ' or '0' for an unpermitted entry so
that the field is hidden during the change transaction. Set the on-line
system response field to a value not equal to ' ' so that system checks
whether there is an entry in the field.
An unsuitable entry would be, for example, '2' in the field control
which would make the field obligatory during change transactions. If
the online-system response field is given an entry 'E', an error
message is always issued for an obligatory field that has not been
maintained.

Error message extract from SAP system. Copyright SAP SE.