Shortly after Apple released iOS beta 6 of iOS 11.4 they revoked the developer profile so non-registered developers were unable to use the profile. Today 24 hours after releasing iOS 11.4 to the public Apple has released iOS 11.4.1 beta 1 to developers. For those who are now unable to install the beta profile there is a workaround, and at the end you will be able to get the new 2.2GB software update.
Go to setting > date and time and change your date back (I set mine 2 months back)
open safari and click here.
Go to iOS 11 beta and choose your phone model click go
Skip the Ad and then click allow
Type in your passcode and install the profile
Go to settings > General > Software update to register you have an update and hit download
Whilst Downloading Go back to setting > date and time and change your date back to automatic
Now go to back to settings > General > Software update and wait whilst your update installs and restart your device
You now have the latest beta installed