Highlights for used fields set again

What causes this issue?

You have unhighlighted fields, this is not allowed.

System Response

The highlighting of the fields used was reset.

How to fix this error?

If you want to delete fields from the communication structure, that are
indicated as 'used' then you must first undo this use. You can do this
by removing the field from the update rules and transfer rules.

Error message extract from SAP system. Copyright SAP SE.