My watch keeps unpairing

What model of phone is it and what version of ios is it running?

What model of watch is it and what version of WatchOS is it running?.

What troubleshooting steps have you tried?.

What errror messages are you receiving?.

Has this watch ever been paired to another phone?(Very important).

Try resetting the watch

Depress and hold the crown and side button until the Apple logo appears

And also force restarting the phone. How you reset the phone depends upon the model phone.

https://www.techradar.com/how-to/how-to-factory-reset-an-iphone