"We are preparing a mailbox for this user"

jyinglingI know this is an old thread but wanted to add a fix that worked for me since this post was the first result when searching the problem on Google.

The Problem:

I was helping a client purchase a new Exchange Online account and licenses for two mailboxes. After submitting the billing info the bank blocked the charge and the tenant was created with an error about being unable to purchase the licenses. The bank was contacted to allow the charge then we logged into the Admin Center and were allowed to purchase the licenses. I applied the two licenses to both accounts but both were stuck with the message "We are preparing a mailbox for this user". I also could not access the Exchange Admin Center or any mail security policies (specifically to enable DKIM on the domain).

The Fix:

I contacted MS support the next day. They confirmed the licenses were valid and applied to both users but both accounts could not create a mailbox. After explaining the initial problem with purchasing the licenses when the tenant was created they suggested starting a Microsoft Business Basic trial license. I removed the Exchange Online license from one user, saved it, then applied one of the Business Basic trial licenses to the user. Suddenly it was able to prepare the mailbox for both accounts (even though the 2nd account was still on the Exchange Online license). I removed the BB trial license, reapplied the EO license and canceled the trial.