Hi there kasryan,
Thanks for using the Apple Support Communities. I understand you’re unable to turn on iCloud Photos on your iOS device. I love using iCloud Photos to keep my photos available to me across my devices and I’ll be happy to help.
Make sure all your devices are completely up to date with the latest OS and you’re signed into the iCloud with the same account across each one:
Update your iPhone, iPad, or iPod touch
How to update the software on your Mac
Once you’ve done that, go to System Preferences > iCloud on any Mac devices you own. Click Options next to Photos, then select iCloud Photos. For your iOS devices, go to Settings > [your name] > iCloud > Photos, then turn on iCloud Photos. Make sure you have a good internet connection and try on both Wi-Fi and cellular data if one gives you issues.
Restart any device if you continue to have issues enabling iCloud Photos and take a look at this link for further help:
Get help with iCloud Photos
Best wishes.