So I was playing a game where my captain Terro was in front of a door that was not open but closed his ability says “Choose a space within 2 spaces. Each other figure on or adjacent suffers one damage and one strain then becomes weakened “ so can I flamethrower people behind doors?

No.

You can't count spaces through a door: it's as if the tiles were completely separated by a black line. Doors also block adjacency so you can't Blast/Flamethrower through them that way either.

Within X spaces is a basic concept in the game. You need to brush up on the Counting Spaces chapter.

(Shortly: like Nick says, you can't be Counting Spaces through walls, doors, blocking terrain, and energy shields.)

Awesome guys thanks for the help !