You might experience an issue marking up a site that requires a username and password to sign in. Even if you enter the correct username and password there might be security controls implemented into your website that prevents it from being accessed within an iFrame - such as the one used to display your site within MarkUp.
If your site doesn't normally require a username and password to access but you are seeing this error, there is a chance your site might require it when it is displayed within an iFrame.
If you experience an issue marking up a site that requires the user to sign in, you'll want to make sure you have the MarkUp Chrome extension installed, as that can take care of some of these types of issues. If you have the MarkUp Chrome extension installed but still can't sign into your site to comment on it in MarkUp, please work with your dev team to see if authentication can be disable for your site temporarily while you work with your team to mark it up.