Brakes and Service Light

I don't know what's wrong but I can tell you that when the check engine light is flashing, it's trying to tell you that there's a condition causing catalytic converter failure to be imminent. When the light is on, there are data and trouble faults stored in the computer which can be accessed by a technician with a scanner. You need to tow the car to a GM specialist ASAP. Here's a directory link for you: https://repairpal.com/buick-repair-in-dallas-texas