There are three main reasons why you may not be able to add new clients.

1. It could be due to the account exceeding the limits of clients available on the plan. See here for details of plan tiers. 
2. Some users have experienced problems adding clients for technical reasons, where the method to fix this issue is clearing your browser’s cache and logging back in. Here is a guide on how to clear cache. 
3. In order to create a client, you need to ensure you use a first name and last name, which are mandatory fields. Please make sure you click “add client” in the bottom right when you have entered the client’s details. If the “add client” is greyed out, please follow the Clear Cache guide in (2).

For a full guide on how to add clients, click this link.