Try exiting and reloading your game. I can confirm the game will let you do this while at the tech selection menu. It says "duplicate technology", which means your only option should be to discard it by clicking the appropriate button. You can't stack multiple copies of anything, even cost discount techs. But it seems your issue is a UI error.
It seems that your screen resolution is too small, and the window with technologies covers the option to acquire technology. Try to set some larger resolution.
Also, download Planetary Annihilation Mod Manager (PAMM) and install a magnification mod. It lets you zoom your scene in and out. It isn't supposed to be played at that resolution, odd as that may seem.
I try to change resolution but I see the same screen. I can't pulse "Rompan filas" (break ranks). I go to try with PAMM.
