Valuation classes in mat. master & & differ: & &

What causes this issue?

You are posting to a material in a previous period. The system has
determined that different valuation classes have been maintained in the
material master record for the previous period (&V4&) and the current
period (&V3&).

System Response

Due to the above situation, the system cannot make the posting.

How to fix this error?

There are two possible ways to post the invoice:
post to the current period instead of the previous period, or
set the same valuation class in the material master for both the
previous period and the current period (this will enable you to post to
the material in the previous period).

Error message extract from SAP system. Copyright SAP SE.