HI all, I need some help in the a trigger. Below is this rule.

*No rental is allowed if a customer's unpaid balance fine exceeds $15.

there are 8 tables in total

Staff=>Branch=>Video=>Rental

Rental_Description=>Category=>Video=>Rental

Customer=>Rental_detail=>Rental

I really need some help I tried my best to run the trigger but always failing. Please guys help me out ASAP.