I have 50 gb, using 46. Phone will not back up In iCloud?

If you have little space remaining on iCloud and your phone cannot back up because it can't find sufficient space you have two options: you can swap to backing up your iPhone to a computer, or you can step up to the next iCloud+ plan which would give you 200GB of space - plenty to be going on with for quite some time.

The pricing for each territory and each plan can be found in the support document iCloud+ plans and pricing - Apple Support