Whenever I try to join a friend's realm, its entrance is denied.

Whenever I try to join a friend's realm,  whenever I'm invited to join someone else's realm, in order to play with a friend, its entrance is denied to me. I've tried all possible solutions, I've seen every available tutorial, but nothing seems to work  it has become impossible for me to play with friends and join other people's realms. 

 

I need help ASAP. 

 

Thank you,

 

Gilberto Sousa