I am also having the same problem, but I have a premium account. I paid for Minecraft and can change my skins to new ones whenever I want. I then followed the instructions to install an earlier version of Minecraft (I used 1.13) and I still couldn't get in the Potter World Mc...
Edit: ok so the problem was the instructions on the website from Potter Word MC. Instead of clicking add server and such, just click direct and copy and past the ip address from step 3 and you're a wizard now. Only works for premium users like me though.