Trying to Invite a User to be an Admin ("Your account cannot be billed automatically")

Hi

I’ve received the following message when trying to invite a colleague to be an “Admin of Entire Account”:

"Sorry, we were not able to add that user.

Your account cannot be billed automatically, so we are not able to add this user. PleaseContact our Support Teamfor assistance."

I’m on a 30 free trial, but have entered my credit card details to move onto a plan at the end of the 30 days. 

Any other ideas why?

Thanks

Emma