Thank you for asking this question.
The most common reason for not being able to add a new source is that you have reached your limit of sources you can add. This can easily be resolved by just upgrading to a bigger plan.
Once you click to add a new source, it will take you to get a bigger plan. Then, you can start adding your sources right away.
If you need to add just 1 more source, then instead of getting a bigger plan, you can let us know on the live chat and we will add enable it for you.