When connecting to your Google Calendar account, you might see an alert about the browser's pop-up blocker blocking the Google Calendar connection window. The message might say; "The window was unable to open, and may have been blocked by a pop-up blocker. Please add this to the list of the sites your pop-up blocker allows to open”.
There are two possible ways in which this can be fixed:
Create an exception for https://app.oncehub.com/. This will whitelist ScheduleOnce with your browser and pop-ups from ScheduleOnce will not be blocked.
You can disable the pop-up blocker entirely.