I have played ATM9 to the sky. And I can't produce flux. I use prism client.

To Reproduce
I follow the step to produce flux dust by place bottom with obsidian air at middle and top with obsidian but also I cannot left click at the obsidian on the top.

I just need to solve it.

Try to do this, but instead of doing it on top of an obsidian block, do it on top of the bedrock block.