Not getting monthly challenges on my Apple Watch

Did you already try to force restart your iPhone and Apple Watch?

Force restart iPhone - Apple Support

To force restart your watch, press and hold the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again.

Also make sure that the "Special Challenges" are enabled in the WatchApp on your iPhone in Notifications/Activity.

If you just paired it back you will have to wait to the 1st of next month to get a monthly challenge.

They only start on the 1st of the month, not the 3rd.