Exit & does not exist.

What causes this issue?

The system could not find form routine &V1& in
<GL:Customizing>Customizing</> Table TFRM.
This may be because the table contents have been 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?

Check your entries.
DEFINE &TAB& = 'TFRM'
INCLUDE 'NA_SYSADMIN_TAB'

Error message extract from SAP system. Copyright SAP SE.