Confirmation control key &1 not allowed

What causes this issue?

You have changed the confirmation control key in a scheduling
agreement.
Either this is not possible, or the confirmation control key entered, (
&V1&) cannot be used.

System Response

You can only replace an existing confirmation control key by another
one if no confirmations or goods receipts have yet been entered for an
item.
<ZH>Acknowledgments of receipt (proof of delivery)</>
If confirmations and goods receipts have already been entered for an
item, you must use a new confirmation control key when implementing
acknowledgments of receipt (LE "proof of delivery" (POD)
functionality). This confirmation control key must be a) identical to
the previous one and b) <LS>POD-relevant</>.
That means that the new confirmation control key must define the same
confirmation categories and these confirmation categories must have the
same indicators for <LS>MRP-relevant</>, <LS>GR-relevant</> and <LS>GR
assignment</>.

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

Hidden text in German does NOT need to be translated. (Writer's notes)

How to fix this error?

Check and change your entries if necessary.
<ZH>Note</>
You maintain the confirmation control key in Customizing for
<ZK>Purchasing</> under <LS>Confirmations -> Set Up Confirmation
Control</>.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.