I had this problem twice, I googled the same article to do the same actions to fix.
Here I wrote down to remember myself.
Error : “paired device unavailable for development” showed in XCode when new to Apple Watch
Fix steps :
- Unpair my watch
- Reboot both iPhone and apple watch
- Pair them again, and setup my watch as a new one
- Plug (unplug first) iPhone to Mac
- On apple watch, say Trust the computer when it asks
- Unplug and plug again
- Close and open XCode
- Open the project and wait a few seconds