It sounds as if you may have encountered a couple of known bugs in the current version of Stalkersoup.
1. The RG6 (and a number of other items) currently disappear when you change maps. The workaround is to spawn in the RG6 when you are on the map where you need to hand it in. (ESC+S spawn menu, WEAPONS tab, LAUNCHER GRENADES section,
RG-6.)
Later on you will need two green military ration packs, which also disappear. For these use the [ESC]+[F4] single-item spawn menu and enter
irp-b,2. The two packs should appear on the ground near you.
2. Kalmyak should appear after you pick up the grenade in the backpack on the floor of the pumping station. Unfortunately picking up the grenade may crash your game. Aleks1970 provided me with a fix for this: a script file called
amk_particle.script which goes in the gamedata/scripts folder. If you get this bug you can download the file from my Google Drive
[You must be registered and logged in to see this link.].