If you are unable to add a publisher to your Articulate Online account, it is likely you have reached your limit of allowed publishers. You can compare available plans to determine if there's a plan that better suits your needs (we're also happy to customize plans if there's not one that meets your requirements).
To correct this issue and get a bigger plan, you can upgrade your Articulate Online account.
If you don't wish to upgrade your Articulate Online account, you can also remove an existing publisher from your account in order to add another (or make an existing user a publisher).
If you are using a trial account, you are limited to one administrator who also counts as the one publisher.