In Commerce the MinQuantity and MaxQuantity are nullable decimals. In carts there is also a check if on if a value is null, to see if the item can be ordered.

When creating a variant through the service api, you need to set a value for the MinQuantity and MaxQuantity. So you kinda need to "fake" it by setting min to 1 and max to a huge value. If set to 0 you cannot order anything.

You should be able to create a variant in the service api as the variant is in Commerce itself