Any version of LetMobile
You cannot enroll your iOS device into LM with either enrollment method (Auto Enrollment or User Portal Enrollment)
There are several user errors that could cause this to occur (I.e. wrong user name, wrong password, wrong domain, etc.) however this is assuming there are not any user errors.
You need have Safari installed and available on your iOS device. If Safari is restricted or not on your device you cannot enroll.
If you are re-enrolling or enrolling a replacement iPhone or iPad, make sure you have deleted the old device record from LetMobile first.