Christos, the pressure bar is wedged up against the section so it does not work to unscrew the section with the bar there. It could damage the sac.

You need to remove the button first, then remove the pressure bar from that end of the barrel. After that you can unscrew the section. Assembly in reverse order.

It can take some practice to slide the pressure bar in past the sac. Sometimes it helps to suck the air out of the sac while doing so. Of course be sure it is empty and the nib is clean.