backup not working

iPhone 11 comes in three capacities: 64GB, 128GB, and 256GB.

Check -> Settings -> General -> Capacity -> xx GB

Here an iPhone SE with 64 GB is not possible to make a backup to iCloud if only iCloud free 5 GB is available.

For that I have iCloud+ subscription.

See this Apple article: Upgrade to iCloud+

Upgrade to iCloud+ - Apple Support

👉You pay monthly for iCloud storage.

The subscription runs as long as you don't downgrade.

If you have upgraded iCloud storage, it would take about 1 to 2 business days in order to process.

Once your request for iCloud+ has been activated, you should receive an invoice in your email box.

See also -> How to back up your iPhone, iPad, and iPod touch

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

-> Then the computer must also have enough space.

Try first to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

Optimize the Wi-Fi connection, see the following Apple articles:

Connect to Wi-Fi on your iPhone, iPad, or iPod touch

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support

Recommended settings for Wi-Fi routers and access points

Recommended settings for Wi-Fi routers and access points - Apple Support

If you need help with your Wi‑Fi password

If you need help with your Wi‑Fi password - Apple Support