If you are a user with access to multiple accounts, your user email must be the same across all sites for you to easily switch back and forth between different site locations.
For example, if you are registered as a team member with @gmail.com at one site and @Marketing Automation.com at another, the sites will not be contained under one account where you can easily navigate back and forth. You will need to log out of the platform and then log in with your other email account.
To easily switch site locations you have access to, simply click your initials in the top right > Switch Account > select Location.