Just found this one out while debugging some code on my test site.
Scenario:
You've copied or moved your existing store to a new space/folder and some images have not been copied over (particularly those that are linked from variations)
Problem:
When selecting variation options for a product you get the error that you need to select options when you try to add to cart.
Cause:
If your chosen variation(s) have an image assigned in the product details to that variation but the image is not available the code fails but critically WITHOUT telling you or the customer.
Solution:
Edit all affected variations to remove images assigned to the variation combinations and/or replace with new source material that does exist.