Investment key &1 cannot be deleted (Investment support posted)

What causes this issue?

Investment support &V1& has already been posted. Therefore, you cannot
delete the investment key &V1&.

System Response

You cannot delete investment key &V1&.

How to fix this error?

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.