If I don't have/want to setup product variants, I don't get the option to "add to card" and I can't checkout either.
Am I obliged to have more than one product variants in order to proceed?
From the official documentation:
Every single product has a master variant, which tracks basic information such as a count on hand, a price and a SKU. Whenever a product is created, a master variant for that product will be created too.