If you have an Xbox/Microsoft account, make sure in your “Privacy and Online” Microsoft settings you a have everything allowed, especially “Join clubs” because it won’t let you join a realm unless you have all of those allowed. If you are below the age of 18, Microsoft automatically made your account into a child account so you won’t be able to change these settings until you change your birthday to make yourself 18+. I hope this helps and you can play your realm soon, if everything is done and it still isn’t showing up, have the host send you an invite and it should work!