Seat belt alarm not working properly

More then likely this is the sensor in the buckle that has failed. What happens when you click the belt into the buckle is there is a switch that gets flipped to tell the module that the belt is connected. When this sensor fails, the module doesn't know and assumes it is not buckled.

I attached the process below on how to change the buckle on a Buick Regal which is similar to your vehicle. Unfortunately we don't have a manual on your vehicle as it is not offered here in the USA.

https://www.2carpros.com/articles/how-a-safety-belt-works

Please review the material and let us know if you have questions. Thanks