unknown extension uasset on UE4.26

hi i want to mod longvinter i want make a custom map withzombie and more but when i try my .pak my character just keep fall cause my plane wont spaw or are too far from the spawn so n the discord server of longvinter they said :

but dont work i can’t import what miko say

i have try this :
Unknown Extension ".uasset" - #6 by ThomasSteine

but nothing…