If you have received quest from Frederyk to kill raiders hiding in Lost Town and you have killed them all in their hideout, but still you can't report it to Frederyk, it's possible you have encountered a bug. In some cases on some computers it can happen that script doesn't register death of one or two raiders. Here's a solution for you:
First make sure that you have really killed all the raiders. It can happen that you have overlooked one of them hiding in some corner. Go to the hideout, turn the combat mode ON and switch your cursor to aiming mode in order to see the character's outlines and check if there isn't living person.
If you have really killed them all and still can't report it to Frederyk, here is the fix - http://resurrection.cz/downloads/raiders_fix.zip
Extract this archive into your Fallout 2\Resurrection\resdata folder. Then in the game, go to the Lost Town (if you aren't there already) and wait about 3 seconds. If you have killed all the raiders, message "You've killed all the raiders hiding in Lost Town." should appear. That way you know it worked and you can go talk to Frederyk.
If none of this helps you, let me know here and upload your save so I can check it. Save files are in Fallout 2\Resurrection\resdata\SAVEGAME folder and each folder there is one save slot.