I bought storage but it is still telling me that I’m low on space. What can I do to fix this ?

That message means that your phone is low on physical storage which is the storage on the phone itself. Buying more iCloud storage does not alleviate the problem. You have to delete some content on the phone to free up space. You can delete previously watched videos and movies, seldom used apps and other content that you don’t need or use.

Take a look at this.