Get an error with setting up a calendar overlay

I have the url for the calendar I want to use for the overlay. But when I enter it, I get the error "Unable to find specified web in the given URL" when I try to resolve the URL. It's valid -- I checked that -- and the error message seems to mean that "calendar overlays from one SharePoint Online site collection to another aren't supported". So that means I can't use a Teams calendar within a sharepoint site?