i only have used 20.1 GB of 50 GB but my phone wont back up as it says there is not enough icloud storage available

Sounds like your phone’s internal storage is higher than your iCloud storage. Go to settings, General, iPhone storage. If that is higher than 50gbs then you need to upgrade your plan.

Hello Nuala1965,

Thank you for reaching out in Apple Support Communities. With the error message you're seeing, we recommend to restart your iPhone. This is a step that can help resolve unexpected behavior. If needed, here's how:

How to restart your iPhone X, 11, 12, 13, or later

1. Press and hold either volume button and the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.

3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.

These steps are outlined here: Restart your iPhone

We look forward to hearing back from you with an update.

Best regards.