Enter a valuation type for material & &

What causes this issue?

You tried to post to a material that is subject to split valuation.
No valuation type was, however, specified.

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

System Response

The posting is not possible, as a valuation type always has to be
entered if a material is split-valuated.

How to fix this error?

Please enter a valuation type.
If this is not possible in the current transaction, maintain a valuation
type in the purchase order to which the item refers.

Error message extract from SAP system. Copyright SAP SE.