You may not maintain an invoicing plan which is active in the document

What causes this issue?

The invoicing plan you have chosen as template is already in use in
another context - for example, as the invoicing plan for purchase
orders in the Materials Management application.

System Response

You cannot maintain the invoicing plan.

How to fix this error?

Specify a different invoicing plan number.

Error message extract from SAP system. Copyright SAP SE.