The error mentions
Server.AuthenticationToken.invalidUserCredentials
Error Message: The username/password specified is not valid.
So you will want to re-enter your log in credentials in the program by going to either:
Windows
Edit > Preferences > Sync
MAC
EndNote Menu > Preferences > Sync
If you are not sure what your password is then you will want to reset the password then go back to the above and re-enter your log in credentials then try to sync.
To Reset the Password:
Please use the ‘Forgot password?’ link underneath the login section. A message will be sent to the email address you used to register with Endnote online that will allow you to reset the password.
If this does not help then Please contact Technical Support directly
1-800-336-4474 option 4 then 1 or
https://support.clarivate.com/Endnote/s/technicalsupport?language=en_US