Hello pvicevic75,
It sounds as though you're attempting to back up to iCloud but being prompted there's not enough available storage. From the description you provided, it sounds as though the backup of your iPhone is larger than the available iCloud storage. If so, you can try to clear the storage usage that's being used currently by following the steps in the "Make more space available in iCloud" section of the following article.
Manage your iCloud storage
While these steps may be helpful, it is likely that you may need to look in to upgrading to iCloud+. The different iCloud+ subscriptions available have different storage capacities. More information about iCloud+ can be found in the article below.
Upgrade to iCloud+
If upgrading to iCloud+ or deleting content currently stored in iCloud isn't appealing, you can always back up your Phone to a Mac or PC. These steps are outlined in the following articles.
How to back up your iPhone, iPad, or iPod touch with your Mac
How to back up your iPhone, iPad, or iPod touch with iTunes on your PC
Take care!