Looking at the image, the door on the right hand side between the terminal and the footpath is not zooned as secure, so the passengers cannot pass through the door; you’ll need to zone this as secure. Annoying, when you place a door on a section of wall, it will not be zoned secure automatically as walls tiles cannot be zoned, so the tile underneath will not be zoned when the wall tile is removed.
Once a departing passenger has passed through a security checkpoint, they can only walk on tiles that have been zoned as secure (red). Equally, until arriving passengers pass through a secure exit, they can only walk on tiles that have been zoned as secure. With this in mind, your passengers cannot use the seats next to the gate counters, or the shops, as they are not within a secure zone.