Why is one of my iPhones unable to back up to iCloud?

I currently am using 2 iphones, an ipod touch 7, an ipad gen 10, and a mac mini, all using the same appleid. One of my iphones will not back up, saying that my icloud is full. I have 50gb, only 26-27% is in use. Why is apple trying to make me purchase extra storage?

[Re-Titled by Moderator]