Hello and welcome to Apple Support Communities Daniellefjd,
We see that you're having an issue with the brightness on your Mac. We know it's important to have your Mac working properly, and we'd be happy to look into this with you.
Let's have you start your Mac in safe mode and test to see if the issue remains. If the issue remains, create a test user and test one more time. The articles below will guide you through those steps, if needed.
How to use safe mode on your Mac
How to test an issue in another user account on your Mac
Let us know how this goes and if you have additional questions.
Thank you for using Apple Support Communities.