No unprocessed message exists

What causes this issue?

You have completed the version of a purchasing document. However, no
unprocessed message exists for this document.

System Response

If this system message is an error message (message type 'E'), you
<ZH>cannot</> save the purchasing document with the completed version.
If the system message is a warning message (message type 'W'), you can
save the purchasing document with the completed version.
<ZH>Note</>
The message type of this message can be set in Customizing for
Purchasing under <ZK>Environment Data -> Define Attributes of System
Messages</>.

How to fix this error?

Create a new message for the purchasing document manually.

Error message extract from SAP system. Copyright SAP SE.