You have tried to substitute a field that cannot be changed

What causes this issue?

You cannot substitute certain fields. These fields are protected in
order to avoid complications with inconsistent data. This substitution
contains a user exit which attempts to substitute a protected field.

System Response

The field retains its original value.

How to fix this error?

You cannot substitute this field. You can display a list of fields that
can be substituted in the substitution editor.

Error message extract from SAP system. Copyright SAP SE.