Well, you can do it with variables.
Go to Control Variables, and notice that one of the options under Operand is actor. You can set a variable to a specific actor's level like that. Similarly, you could add up all the levels of the party or check each one individually if you want the level requirement to be for the entire party. Anyway, once you set that variable to the level of the actor, you can use a conditional branch and make it so that variable has to be greater than or equal to a specified number. That way you set a level requirement.