I can not back up my iPhone even though I bought more storage

You cannot buy more storage for your phone. It will always have the same amount it came with. You have bought space in iCloud, Apple's syncing service. To create space you have to delete photos, messages, apps, etc.. Check these links.

https://support.apple.com/en-us/HT20165

Clear Other storage on your iPhone or iPad - Apple Support

review this link and see how much available space you have in iCloud..

Manage your iCloud storage - Apple Support

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

FYI, the first link you provided comes up for me as 'Page not Found'

Thanks for telling me about the link.. I shall take care of that...