Structure change at field level (convert table &)

What causes this issue?

The structure of the table in the ABAP/4 Dictionary changed at field
level, e.g. due to deletion of fields or a change in the primary key.
This structure change in the ABAP/4 Dictionary makes a conversion of the
table structure already created in the database necessary for reasons of
consistency. The data is unloaded from the database table, the new table
structure is created in the database and the data is loaded back to this
new 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?

Error message extract from SAP system. Copyright SAP SE.