BAdI: Field & not ready to accept input (Change will not be adopted)

What causes this issue?

The contents of field &V1& have been changed in
<DS:GLOS.5C733369F735D311AE620060B03C9BBE>BAdI</> <LS>MB_MIGO_BADI</>
(method <LS>LINE_MODIFY</>). The field, however, is not ready for
input, so the change cannot be copied.
In method <LS>LINE_MODIFY</>, only the contents of fields that are
ready for input can be changed.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.