Cost element & can only be used for activity inputs

What causes this issue?

You have entered cost element &V1&.
This cost element has category 43 and is therefore used for internal
activity allocation or for plan activity inputs.

System Response

Cost element &V1& is ignored by the system. The Bapi is not processed.

How to fix this error?

Remove cost elements of category 43 from your planning.

Error message extract from SAP system. Copyright SAP SE.