There is a minimum resolution requirement for each product when you are using Lab-Fulfillment. This is to prevent low resolution photos from being sent to the lab which will result in poor quality. If a product is not available for purchase but it has been added to the price sheet, it means the photo you have uploaded has not met the minimum resolution requirement.
For example, a 16x20 requires the photo to be at least 1024x1280. You can see example minimum resolution requirements here:
Lab Fulfillment: What is the minimum resolution required for products?
It might also be helpful to check that you have the correct price sheet attached to the collection:
How do I change the price sheet attached to a collection?