No pollination.

If pollination is an issue for you because you're growing in a greenhouse, you might want to try a parthenocarpic variety in the future.

For the time being, i'd make sure you are doing the pollination correctly because that's likely the problem.

May be calcium deficiency? It gives blossom end rot.

Are these the first fruits they are putting out? I've heard that sometimes they just abort the first few.

hand pollination is almost 100% efficient. I can stir a small paint brush inside a male flower, pull it back, and see the pollen all over it.

It's not the first fruits, they've been doing this for over a month.
I use a soft artist's paint brush to pollinate, same as I have for the last 4-5 years. Previous years yielded lots of zucchini.

I'll look into calcium deficiency. But the plants themselves are so healthy!

I use water from our tilapia tank, so it has no added chemicals except the lovely fish-added fertilizer.