Max one history lesson on 6th position in each class (solution 2)

You can add advanced cardrelationship Max periods per week and specify the 6th and 7th period in the time map:

Do not forget to set "apply to selected subjects in selected classes".

The software will then check max 1 period of History in each selected class. If you for example use Apply globally the software will allow only one history on 6th or 7th in the whole school.