Iphone not backing up even after buying storage and turning off photos

I am a new iphone user. Since I have around 100 GBs of photos on my device. I turned off sync to iCloud for photos from settings. I also upgraded the iCloud storage to 50 gb. Still, it shows not enough storage for backup and fails. It’s only showing 3.1 gb used out of 50 gb in Icloud but still not backing up. Please help