Sometimes Safari can prevent users from signing in to StudioBinder. This can easily be solved by clearing old site data that's stored in the browser.
Just follow these instructions:
In Safari's menu bar, click Safari > Preferences
Select the Privacy tab
In the Cookies and Website Data section, select Manage Website Data
In the pop-up, search for StudioBinder
Select StudioBinder row and click the Remove button.
After that, refresh StudioBinder, and try signing in again.