Use the user exits correctly

What causes this issue?

You want to use a conversion with user exits. This message could be
due to the following:
a) The user exit can only exist in a field movement if the sequence
number in the field movement is initial.
b) If you use a user exit, the sender field must be filled in all field
movements that refer to the same receiver field.
c) The structure for the user exit name is 'Uxx', where 'xx' represents
a two-character number.

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?

Note the above requirements.

Error message extract from SAP system. Copyright SAP SE.