Enter a priority for the field marked as free

What causes this issue?

The field with access type 'A' has been given no priority or priority
0.

System Response

The fields in the free key part need to be evaluated so that the system
can choose these condition records in this access. You evaluate the
fields by entering a priority between the values of 1 (very high) to 99
(very low).

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.