Back-posting only poss. w. same val. class (cur. mnth &, prv. mnth &)

What causes this issue?

You are posting a goods movement for the previous period. The system
has discovered that different <GL:valuation_class>valuation classes</>
have been maintained for the previous period (&V2&) and the current
period (&V1&) in the material master.

System Response

The situation described above is not allowed and the posting will be
rejected.

How to fix this error?

There are two possible solutions:
the valuation classes in the material master record must be made the
same,
or,
you must post the movement to the current period instead of the
previous one.

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.