Not possible to delete entry

What causes this issue?

A condition header with the condition header type "Contract" or
"Contract fixed" can not be deleted if the locking-in period in the
header is in the past or is currently valid.

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?

Error message extract from SAP system. Copyright SAP SE.