Use the user name (not the email address). The user name can be found in the top right corner after you have signed in to Reddit Ads:
Reddit ads users can have a few different permission levels:
Funnel will work with any level but it is good practice to create a separate user with low permission level (Analyst) and connect that.
Connect a user that has access to several ad accounts
If you have access to multiple ad accounts via the Reddit account you're connecting, please enter the name of the account you'd like to retrieve data from, exactly as it appears on the "Switch User" page, select one of the names in the "users" tab.
Funnel does not support connecting with Two-factor authentication. We recommend creating a separate user with permission level Analyst and two-factor authentication disabled.