Enter at least one assignment with a contract reference

What causes this issue?

Message number M8_251

System Response

You want to post directly to a G/L account or material account.

How to fix this error?

Enter at least one assignment for a purchasing document, so that the
system can determine the supplier. To do this, choose the <ZK>Contract
</> tab page and enter the document number. At least one contract item
must be selected for settlement before you can post to G/L accounts and
material accounts.

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

Choose the <ZK>G/L Account</> or <ZK>Material</> tab page and enter the
required data.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.