Before you can log in to EndNote Web you need to register to sync your desktop and online accounts:
Launch EndNote 21 > Go to Edit menu > Select "Preferences".
Select Sync > Choose Enable Sync. You will see a small window that pops up, click on Sign Up
Re-enter your email address in these two fields and click Submit
Enter the password for your EndNote Online account.
Verify your information and click “I Agree” on the EndNote® End-User License Agreement. A message will appear indicating that the "EndNote registration is complete."
Click on Done and that will activate the EndNote program with your email address > Click ‘Apply’ > OK.
Then, go to the ‘Library’ menu at the top toolbar of the EndNote 21 > Select ‘Sync’.
After you have synced your EndNote Library, you can access EndNote Web: https://endnote.com/weblogin/