Coolant doesn't heat after driving car and coolant light staying on

Thermostat open or overheating safe pop off thermostat will open if a overheating occurred, so check that.

You have 2 separate issues. 1. Replace thermostat, it's stuck open not allowing the coolant to to heat up. 2. The fluid light staying on is a common problem with the GM's. The float in the reservoir sticks. Remove reservoir and see if you can rinse it off with soap and water and shake it to break it loose, if not, replace reservoir, it comes with the level sensor included