Hello littleBlackcloud,
Based on your post it sounds like you’re unable to save Reminders on your Apple Watch as expected. I’m happy to help you isolate this issue a bit further.
Have you tried restarting both your Apple Watch and iPhone one after the other to see if that helps? If so, are you able to create and save Reminders on you iPhone as expected?
If you create a Reminder on your iPhone, do you have access to the Reminder on the Apple Watch? When you create a Reminder on any of the devices can you see the Reminder signing into iCloud using a web browser? This can help you isolate the issue to the Apple Watch as well.
If this turns out to be isolated to the Apple Watch, you can try unpairing the Apple Watch and setting up again to test, as a new Apple Watch. Take a look: Unpair and erase your Apple Watch
Keep me updated on your findings.
Cheers!