The information in this article can help you when you receive the message "None of the users in this session are members of Nintendo Switch Online. Please enjoy Trial Mode. Your opponents will be CPU controlled." when trying to play online in Nintendo Switch Sports.
What to do
Power the Nintendo Switch down completely.
Putting the system into Sleep Mode isn't sufficient - you will need to completely shut down and restart the system.
Turn the system back on and launch Nintendo Switch Sports from the HOME Menu.
Select Play Globally or Play With Friends.
When prompted to select your user, select a Sportsmate that is tied to a Nintendo Account with an active Nintendo Switch Online membership.
If you're not sure if an account has an active membership, you can check the current membership status.
If you have a Nintendo Switch Online family membership, be sure that all of the users have been added to the family group.
The game should now recognize your active membership and allow you to proceed.