Why does my iPhone keep giving me the message my storage is full, when I’m paying for the cloud? It shows I’m using 17.3 GB of 50 GB

Because iCloud storage is completely separate from iPhone storage. The purchase of iCloud storage does not increase the storage of the iPhone. iPhone storage is fixed when you purchase it. The only way to slightly increase iPhone storage is to delete content from the iPhone itself.

The message indicates that the storage space on your phone is full. Having cloud storage space does not change available space on the phone.