i cannot turn my watch off

Apple support's article:

Turn on and wake Apple Watch – Apple Support

For watch OS 5: says:

• Turn off: Normally, you’ll leave your Apple Watch on all of the time, but if you need to turn it off, press and hold the side button until the sliders appear, then drag the Power Off slider to the right.

Tip: You can’t turn off your Apple Watch while it’s charging. To turn off your Apple Watch, first disconnect it from the charger.

...and if that does not work try Forced Restart (i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo. ) then you should be able to turn it off.