The layers button is disabled because you are already using 1 layer. Currently maps only support 1 layer. We are currently prototyping this capability but we do not have a time-frame just yet. You can however join and check out Qlik Branch where developers have built extensions. You could request or inquire about this capability.
Please mark the appropriate replies as CORRECT / HELPFUL so our team and other members know that your question(s) has been answered to your satisfaction.