digits app not syncing

almost positive the other device would have to accept it on their end or deny it.

They never had the app. Even when it was working correctly 

did the S10 user delete the digits app from the phone?

Curious that it was working and now it is not. Here are a few things to start things off. 

• Is the S10 possibly using a third party app to send and receive messages? If they are using Whatsapp or Hangouts or others like that, those messages would not sync with the DIGITS app. 
• In the DIGITS app on the S21, tap messaging and then use the drop down at the top of the screen to make sure the correct phone number for the S10 is selected. You might have the app filtering to the wrong phone number. 
• Make sure that both phones and the DIGITS app are all fully up to date. 
• Sign out of the DIGITS app and sign back in. 

I have a Pixel watch and phone and I can sync through the watch app but I only have 1 phone linked.

The one with the app is a s21. The one phone that I want to see messages from is an s10. I restarted both phones and Uninstalled the app too. 

Have you tried rebooting both the phone and watch?

What type of phone and watch.