Why isn’t my iPhone backing up

See this Apple article: How to back up your iPhone, iPad, and iPod touch

How to back up your iPhone, iPad, and iPod touch - Apple Support

Tell us: back up to iCloud or to your PC?

If you turn On iCloud you get 5 GB iCloud free storage.

Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.

If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.

👉 Check iPhone type and how much GB storage.

For example if you have an iPhone with 512 GB, you need iCloud+ subscription 2 TB.

If you need more iCloud storage, see this Apple article: Upgrade to iCloud+

Upgrade to iCloud+ - Apple Support

See also: iCloud+ plans and pricing

iCloud+ plans and pricing - Apple Support

• If you have upgraded iCloud storage, it would take about 1 to 3 business days in order to process.
• Once your request for iCloud+ has been activated, you should receive an invoice in your email box.
• If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.

Back up to your PC you need to free up space too.

You need to have a good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

See if anything here helps

Backup couldn’t be completed.   -

Backup couldn’t be completed or you can’t restore.  *

Backup couldn’t be completed or you can’t restore (2).  *

Backup couldn’t be completed or you can’t restore (3).  *