What version is the realm on? I had a similar problem when trying to join my sister's world when she was in Xbox and I was on mobile. It turns out she'd been automatically opted in to the beta version, so she was already in a beta version of 1.16 before it was released. she ended up having to opt out of the beta so I could join, because I wasn't playing on the same version. Make sure nothing like that is happening with your friends realm, and double check that you're both on the same version.
What version is the realm on? I had a similar problem when trying to join my sister's world when she was in Xbox and I was on mobile. It turns out she'd been automatically opted in to the beta version, so she was already in a beta version of 1.16 before it was released. she ended up having to opt out of the beta so I could join, because I wasn't playing on the same version. Make sure nothing like that is happening with your friends realm, and double check that you're both on the same version.