Terms of payments in purchase order & differ from current terms

What causes this issue?

The terms of payment in the current invoice differ from those in the
current purchase order. In purchase order &V1&, other terms of payment
were stipulated:
IF &V2& NE ' '.
Terms of payment key &V2&
ENDIF.
1st cash discount period &V3& days &V4&%

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

Either you have selected more purchase orders with different terms of
payment, or you have changed the terms of payment manually.

System Response

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.