You may not delete an append line (refer to long documentation text)

What causes this issue?

You cannot delete the append line because the assignment of an append
structure to a table is an attribute of the append structure.

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?

If you want to delete the append structure, enter the name of the
append structure in the initial screen of the Dictionary or in the
Repository Browser and choose 'Delete'.
Deleting an append structure automatically results in reactivation of
the appending table or structure.

Error message extract from SAP system. Copyright SAP SE.