Your organization must have an available user license in order to create a new user in TotalABA. There two ways to easily determine if there are any available license.

The recommended way is to go to the Service Orders tab on the support page and click on the Company Information link.

This will open a new browser tab. Check if there are any remaining Salesforce Platform licenses (see figure below for an example where 1 license is remaining). This will show how many new users can be added to your site.

The second way to determine if licenses are available is when creating a new user. If Salesforce Platform is not an available option for the User License field, then there are not any available licenses.