Do you currently have an Behance account with the same url? If so, you’ll want to change the username on that account to free it up for your Team:
To do this, log into your existing account and go to Account Settings (https://www.behance.net/account/settings). Under Change URL, put in a new username--it’ll be best if it corresponds to an admin associated with the team account, so you may want to put your own name here.