I have 5 GB of storage and only use less than 1 GB, but but it says I cannot back it up because I do not have enough storage available.

Which iPhone do you have?

What is the storage capacity?

See ⤵️

What's the difference between device storage and iCloud storage?

What's the difference between device storage and iCloud storage? - Apple Support

Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support

iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support

Manage your iCloud storage -> Manage your iCloud storage - Apple Support

An example:

An iCloud plan is available in 50 GB or 200 GB or 2 TB.

iPhone SE 3rd gen is available in -> 64 GB, 128 GB or 256 GB.

To back up an iPhone SE 3e with 256 GB to iCloud, a 2 TB iCloud subscription is required.

If you back up to your pc you need space too.

On your phone......

Tap Settings

Tap General

Tap iPhone Storage

What does it say right next to iPhone near the top of the screen? Something like xxGB of xxGB Used will be displayed.

Post back what you see for xxGB of xxGB Used.