Based the gmail account the registration did go through so you should be able to sync.
If you are getting an error you may need to re-enter your password or maybe reset the password.
If that does not resolve the issue:
Please contact Technical Support directly
1-800-336-4474 option 4 then 1
or
https://support.clarivate.com/Endnote/s/technicalsupport?language=en_US