If you are creating a new user in a Small Business portal, there may sometimes be delays in provisioning the mailbox. This won’t be visible from the admin portal. If you immediately try to add an email alias, you will receive the message “Sorry, we can’t make changes to email aliases right now. Please try again later.”
You are receiving this message because there is no mailbox to add the alias to, even though there is an account created.
Allow some time for the mailbox to be provisioned for the new user, approx. 1-2 hours.
If you want more visibility to see if the mailbox has been provisioned yet, log into https://outlook.office365.com/ecp using an admin account for your Small Business tenant.
Look in Mailboxes for your new user’s email address. If it’s not present, either wait for a few more hours or contact Office 365 Support for assistance.