Apple Watch workout

The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

I have also heard that according to Apple Support, for the activity ring to register your workout (for outdoor walk on exercise ring) you need to maintain a speed of approximately 3.3 mph or faster and increase your heart rate to approximately 75% of your High Heart Rate (from the iPhone Watch > Heart app). This definition of a "brisk walk" is to encourage you to put more effort into the walk, If you slow down or pause, the watch will stop giving you exercise credit, It will still measure your steps and calories burnt but will not give you any credit towards the exercise ring or workout app. A brisk walk speed is different for everybody as the watch takes into account your age, sex, weight, and heart rate which is why it is important to calibrate the watch and have the correct details set up in the Health app,

I can't vouch for any of those specific numbers, but I'm sure there is some kind of criteria. See the "Make sure that you earn Move and Exercise credit" section in:

Get the most accurate measurements using your Apple Watch - Apple Support

You should also make sure your watch is calibrated:

Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support

Also, see the solution to:

apple watch series 5 activity recording n… - Apple Community