In order to be able to create codes, you must be added as a User of the group the prefix is in.

If you have User access to the group and still cannot create codes, the prefix may not be associated to the group and/or the billing account has not been set up. See Create an Account for Existing Prefix for instructions on how to create the billing account. 

If the prefix is already accessible in the group and there is already a billing account, please contact Client Success for further assistance.