Despite adding more storage = 20GB my devices tell me that I can not take any photos/video, why would this be?

Because the issue is that you don't have sufficient available space on your phone, not that you don't have enough space in your iCloud account. Adding storage in iCloud does not give you more space on your phone.

You can check your available space by going to Settings>General>Usage. You'll need to delete or sync some data off your phone to freee up more space.