It is important for the correct users to have access to your websites in Kademi. Access can be restricted for groups with website access, which is additional to permissions - these still need to be set up for each group.
When using the 'Login as' feature, you will only be able to log in as a user if they are part of a group with access to the relevant website.
To set up website access for a group, go to Website Manager, then Websites, and open the settings for the site that the group should have access to.