Hey Nickac8200,
If you are having issues with your being unable to add Reminders on your iPhone, you may want to try some things to troubleshoot.
First, quit all running applications and test again -
Force an app to close in iOS
Next, I would try resetting the iPhone -
Restart or reset your iPhone, iPad, or iPod touch
If the issue is still present, you may want to restore the iPhone as a new device -
How to erase your iOS device and then set it up as a new device or restore it from backups
Thanks for using Apple Support Communities.
Be well,
Brett L