I have bought space 50gig but my iPhone refuses to back up , forces me to delete almost all my apps now I can’t access my WhatsApp

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.

You pay monthly for iCloud storage.

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.

The iPhone 7 Plus storage 32 GB, 128 GB or 256 GB

iCloud storage with subscription -> 50 GB, or 200 GB or 2 TB.

If you turn on iCloud, you get 5 GB free iCloud storage.

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

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

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

If you back up to your computer, you need to have enough space too.

See also -> Manage your iCloud storage

Manage your iCloud storage - Apple Support