Thank you for reaching out to the Community and telling us about your issue, though I apologize that you are experiencing this.
What exactly are you seeing after you try adding the account? What message does Quicken display?
You can use the link provided in Quicken to visit the site you are attempting to log in to. So you click on the betterment link, it opens a web page, and you should try signing in there. I have provided a screenshot of that link found in your software below.
You must be able to log in on the page the link takes you to in order to successfully log in using that instance in Quicken.
Let us know how this goes.