The save twice options works, even though the user does not appear in the list. This is because they are first invited to join the enterprise account via an email from BOX. Once they accept they will show up in the list. They need to click on the link in the email

I'm experiencing the same issue and need resolution ASAP.  I've tried the suggestions above without any change in results.

I've been stuck with this issue for over a month. I've tried all of the above and nothing is working. Can someone from the Box team please contact me to help troubleshoot? 

I am dealing with this as well. The user deleted the account they made, but I'm still getting the error when I try to invite them to our enterprise account.

I am having the same issue. The user deleted their old account, but it still will not let me add them. 

Solution is here: https://support.box.com/hc/en-us/articles/32163598260371-Inviting-Unmanaged-Users-to-Become-Managed-Users-in-Your-Organization?q_mailing_7TY4AeL8XdqWzaJp2wXg1aTZDmSMwtomsFcnh=Rp3hogU4NgxJrpfYkWqdSsFT7wcnJLdM7LAHcz66b2NMqa4BQ8cCe5Z9a

Same issue here.  I deleted the users account a week ago and I still cannot added them to our managed account!  What is the solution?  Impossible to get ahold of support.