You should have a dialogue option to get the item back, you don't buy it back.

There's no option to get the item back,his just self-introduction all the time

Do you have any other mods/maps installed? There may be a conflicting ID.

Dialogue option has ID too?

All NPCs use IDs, so if you have another map or mod installed that also has NPCs (Elver, Arid, Escape from Unturnov, etc), they may conflict with Kuwait.

I don't know how too check up ,but there is item ID conflict exactly

From Steam, you can check your subscribed items through the workshop, If you've got anything besides Kuwait, check if it has NPCs or not, if it does, try disabling or unsubscribing from it. If nothing shows up under Steam, you can try checking through your File Explorer under steamapps>workshop>content>304930.
You should be able to see a lot of numbered folders that relate to different workshop items, do the same here and check if its any maps or mods that could have NPCs.

I unsubscribe a lot of mods,it works now

Do you happen to know what mod causes it? If not, no worries.

for me flora was the only npc whos quest thing was broken, i uninstalled a bunch of mods (Hawaii Assets, Ural Mountains LN, Ural Mountains LN Content, Japan, Arid, FIFFS Objects v1.5 FIXED, Downtown Survival, PEI v3's Custom Content, High Speed Race Reloaded, Festive PEI, Russia Festive, Deepwood) and now flora finally works, but the other 3 npcs are broken

:(