&-structure: invalid change: length of field & was changed

What causes this issue?

For a BAPI or IDOC structure, you have attempted to change the length of
a field. The structure, however, is marked as "frozen". This means that,
in this version, it is delivered as productive in BAPIs and IDOCs,
respectively. Renewed delivery with the peformed change would contradict
the agreed interface.

System Response

The activation is rejected.

How to fix this error?

Take back the change. If it is absolutely necessary, you wil first have
to reset the "frozen" status. For information on how to do this and what
you need to watch out for, refer to Note 1297242.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.