Hi
@roxyrose!
Are you joining to the server on a premium Java minecraft account?
Make sure you are using the IP
play.potterworldmc.com
Make sure you don't have any spaces before or after the IP too!
If all that is still not working, try checking if you have a Firewall currently enabled. Try temporarily disabling this as you try to join.
Hopefully you can get onto the server soon, and let me know if this still isn't working for you!