Is it possible that your backpack was full when you were given the task? :o If that is the situation, the chair may be lying around the world somewhere or you may be softlocked. If the chair is lying around somewhere on the map, I would recommend backtracking to the location you were when given the task and searching/walking around there to see if you can pick it up. If it has disappeared from the world, you may be need to restart without saving to avoid the softlock, but considering how long it has been since you posted this, you may have already saved. :S It may be possible to contact the developer on the discord, but I am not sure.

There may be other options, but I am not sure what they are or how to accomplish them. :S

Thanks for answering.

Yes, I thought about restarting the game, but there are 35 hours left. The truth is, I don't know where the chair went missing. It's my son's game, and he can't remember how he lost it, hehe.

I'll keep looking.

Thank you very much!
Best regards!

You are welcome and I am sorry if I am not more help. ^_^; If you can find the dev's contact info, you may be able to send them the save's logs and they may be able to find out what happened for sure, and may know if there is a way to fix it without restarting.

Edit: I found this thread for contacting the developers if that helps. :D
https://steamhost.cn/steamcommunity_com/app/824000/discussions/0/4353369456409811565/

Thank you very much!
Best regards!

You are very welcome! :D I hope they can help. ^_^
Best regards to you too! ^_^