To use checkra1n with untested iOS/iPadOS/tvOS versions, open checkra1n.
Click the button that says Options.
Now tick the box that says "Allow untested iOS/iPadOS/tvOS versions". Then click Back. Now your untested iDevice should be able to be used with checkra1n. This can be dangerous, don't use this unless you know what you're doing. This ticked box won't stay ticked when you re-launch checkra1n on your computer.