Apple Watch message: "Sign Into Complete Purchase"
Here is the fix! 


Open your Apple Watch app on your phone and go to “My Watch”
Go to Settings/General &amp; turn off Automatic App Install. 
Now go back to “My Watch” and scroll to the bottom to see “Available Apps”.
Any apps you see that are trying to install are the culprits causing the notifications!
Delete those apps from your phone, and if necessary reinstall them with your Apple ID used currently.  
Turn Automatic App Install back on if necessary. 


You now won't receive any more of those annoying notifications!