Tap Install to overwrite your current version with the beta version.
Steps to reproduce (select examples)
TestFlight says the YouTube beta is not available in your region. Fix: This is rare but happens. Ensure your Apple ID region matches the invite link region. You cannot bypass this without changing your App Store country, which is not recommended. youtube beta testflight upd
Google hosts a public invitation link for the YouTube iOS beta. Unlike smaller developers who post links on social media, Google manages its slots dynamically. Tap Install to overwrite your current version with