like on picture, in previous game I saved design of interwar medium tank that I want to share between games, I researched project for flame tanks, and it says "you require the following modules" which is empty, and design is not possible to use?

design that I have saved in file was complete though and I was trying to load it, but it does not let me?What you are showing is an incomplete design.
You do not have a weapon in the design. If you are trying to design a flame tank, you need to place a flamethrower in the design and designate the design as a Flame tank.