Before you can create products, you'll want to enable at least one (and preferably all) payment method for your store. Like most things on SellApp, it's super easy to do so.
Log into your account and navigate to your store dashboard. It'll probably look like this when you just signed up.
Click on the cog icon that's found at the top of the sidebar. That will take you to your store settings. On the store settings page, click on the "Payment" tab to display the payment methods that are enabled for your store.
On this page, you can add the relevant payment methods to your store. Once you've added/enabled at least one payment method, you will then be able to create products.
Don't forget: The more payment methods you offer your customers, the higher your conversion rate.