What causes this issue?

There is an inconsistency in the main flow.

System Response

The total amount of the main flow is calculated from the sum of the
quantity and price in the contract. The total amount that you entered
does not match this value.

How to fix this error?

Adjust the amount accordingly.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.