This error indicates that the username and password for your EndNote online account are incorrect in the EndNote sync preferences. To update your credentials follow the instructions on the EndNote support page

Your EndNote online account password is the one you created when you first started using EndNote and is specific to EndNote. You may have created your account using your University e-mail but the password you selected should have been different to your university one. If you have changed your university password recently it will therefore not update your EndNote one.