Append structure & cannot be appended to table of type &

What causes this issue?

The specified append structure was defined for a table which is neither
a structure nor a transparent table.
Appends are only permitted for the above-mentioned types of tables.

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?

Change the table type or delete the append structure.

Error message extract from SAP system. Copyright SAP SE.