Can't access friend's realm

So my friend got realms on his PS4 and he sent me an invite to my Microsoft account (I have a PS4 too). I can see the invite but when I click accept it says I can't access it due to how my Microsoft account is set up. When I click on the "help me fix it" it takes me to a website telling me how to fix a child account to be allowed to play realms on a Xbox. Now I do have a child account on PS4 but, my Microsoft account isn't.