How do I fix MacBook Air stuck on startup support page?

Go to the link provided in your error message as it points to an Apple article with instructions on how to proceed.

FYI, here is an Apple article with various error screens you may encounter when attempting to power on & boot a Mac:

If your Mac doesn't start up all the way - Apple Support

Try restarting your MacBook Air by holding down the power button until it shuts off, then turning it back on. If it still gets stuck, you may need to boot into recovery mode to troubleshoot the issue.