You cannot remove pymt plan type from contract &1; An active plan exists

What causes this issue?

You tried to remove a <DS:GLOS.CE3D29A77D72D411BA570800060D888B>payment
plan category</> from contract &v1&, although an active payment plan
still exists for this contract.
This action would result in the
<DS:GLOS.2A7E8A9C5C28D311B60F0060B03C2BFF>payment plan</> being
terminated incorrectly.

System Response

INCLUDE ISU_VERARB OBJECT DOKU ID TX

How to fix this error?

Delete the starting month from the contract to terminate the payment
plan.
The payment plan will then be terminated automatically during the next
invoicing run.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.