There are three possible reasons why you can't create any more brand themes:

1. The functionality to create multiple brand themes hasn't been activated for your account. For more information about how to create multiple brand themes click here.

If this is the case, when you click on Brand theme from the Customisation dashboard, you'll be directed to the main brand theme dashboard.

2. The functionality to create multiple brand themes has been activated for your account but you have run out of brand theme credits, and need to purchase more.

If this is the case, when you click on Brand theme from the Customisation dashboard, you will be directed to a brand themes overview page, but there will be no button to Add brand theme.

3. Your user role does not provide you with access to this functionality. 

If this is the case, when you go to the customisation dashboard,  the brand theme icon will be greyed out.