Hello, I am a young aspiring Ukrainian writer and when I listened to the retelling of your game on YouTube, I was delighted. I decided to write a book about your game but with changes and additions to the plot. I want to ask you for permission to use the universe of your game "Metal Garden".
Hey there. If what you're planning to write is just online fanfiction that you share with your friends, then of course there's no problem with that, you don't really need my permission. However, I can't endorse an officially or commercially released book set in the Metal Garden universe, as that would cause a number of legal issues for me.
Also, speaking as a fellow creative, I'd hugely advise against situating your own large works in other people's IPs and settings. Making a game or writing a book is a ton of work, and you don't want access to it to remain at the whim of an IP holder.
If you're really set on writing this book, I would suggest making your own distinct universe and situating your story within it. Not only will this mean you own your book in it's entirety, but it will also mean you get to make any changes or additions that you want since you'll be working from a fresh base.
Hope this was able to answer your question.