Variable product cannot be added to cart

Hello!

I created a product called “Bopo bundle” and has 3 bundle items in it. The 3rd item (#300 Panceta, vakuumiran kos) is a variable product. When I hit the Add to cart button, the message appears: “Panceta, vakuumiran kos – 0,3 – 0,4 kg” is invalid variation. You cannot add this bundle to the cart.

Interestingly, if I added other variable product (#791 Darilni bon), everything works = addition to the cart is correct.

Could you please explain me, what have I been doing wrong? Are there any settings in the variable product that are causing this error?