Another user posted the solution to this. Sorry I forgot who but I copied the description so here it is:
you have to edit a config file using notepad.
its located in gamedata/configs/misc file is named stalkers_upgrade_info.ltx. open it and press ctrl+h then in the first field type "false, false" and in second field "false, true" then press "replace all". that worked for me
Gefafwisp
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