You don't have enough free space to download an update. For iOS 14.8, you'll need 3+GB of free space. To go to iOS 15, you'll need at least 6GB of free space.
When you bought storage, you did not add space to your phone. iPhone memory cannot be increased. The storage you bought is iCloud Storage, which permits you to sync more data with iCloud, but again, will NEVER increase the space on your phone.
You are going to have to delete things from your phone you don't need or use. This can include apps, photos, music, videos, movies, messages, files, email.
You can if you want connect your phone to your computer and update it using iTunes