Choose a valid plan version

What causes this issue?

In the "Plan version" display frame, you did not select a valid
version.

How to fix this error?

If you wish to maintain or display plan settlement rules, choose
"Plan settlement" and select a valid plan version.
If you do not wish to maintain or display plan settlement rules, then
acknowledge the error message with "ENTER" to
continue maintenance or display of actual settlement rules.

Error message extract from SAP system. Copyright SAP SE.