Cannot make this change to the editing mask (see long text)

What causes this issue?

You want to change an existing coding mask, or want to create a new mask
and have chosen a key for which a coding mask already exists.
You can only change existing masks as follows:
You can change a numeric section to an alphanumeric section of the
<ZH>same length</>.
You can extend the coding mask with an <ZH>alphanumeric</> section.

System Response

INCLUDE 'NA_ALLG_NOFUNKTION' OBJECT DOKU ID TX

How to fix this error?

Change the coding mask as described above or create a new coding mask
with a key that does not yet exist.

Error message extract from SAP system. Copyright SAP SE.