Tax code & from PO & & replaced by &

What causes this issue?

In purchase order item &V2& &V3&, the purchasing department has entered
the tax code &V1&. But in this invoice you are using tax code &V4&.

System Response

The system will replace tax code &V1& in the purchase order with tax
code &V4&.

How to fix this error?

Check this data. Note that tax postings may already have been made on
the basis of the tax code in the purchase order at goods receipt.
If necessary, change the tax code in the invoice (by overwriting the tax
code in the supplier item).

Error message extract from SAP system. Copyright SAP SE.