Selfhosted

Subnet routing is generally far more complex than simply installing the client. If you aren't succeeding at one you're likely not going to succeed at the other.

I don't know the exact problem based on what you've described and I'm not going to promise I can solve it for you but I'm going to try to give you some tools you can use to help yourself a little and hopefully be able to better understand what is going wrong and that will help you understand what you can do about it. Don't get frustrated by this issue, this is a learning experience and this is a skill you need to invest in and develop so that you're not just blindly copy-pasting instructions from videos (which is a bad place to be)

Step 1: Figure out where your tailscale.sh actually is.

Once inconsistency I noticed in your description of what's going on is that you're attempting to run tailscale.sh but you're describing a path of /home/deck/documents/github/deck-tailscale.sh not sure if this is just a typo or what but that describes a file called deck-tailscale.sh which is not the same thing as tailscale.sh.

I think the repository you've downloaded based on those instructions is called deck-tailscale however a repository is a folder full of files, and tailscale.sh is ONE of those files. That repository's name would probably be /home/deck/documents/github/deck-tailscale/ so if you're looking for tailscale.sh inside that repository it will be /home/deck/documents/github/deck-tailscale/tailscale.sh. (two tailscales in the full path, one for the repo and one for the file itself)

You can verify all of these paths by using the ls <path> command, ls (that's L and S, not IS) means "list" and is similar the dir command in Windows, it will show if the file you specify exists, or if it is a directory it will list all the contents of that directory. ls is a useful command to explore the directories and see which ones exist and which ones don't. You can work your way up the path to see where things are going wrong, for example, if ls /home/deck/documents/github/deck-tailscale/ does not exist, try ls /home/deck/documents/github/ and if that doesn't work try ls /home/deck/documents/ and so on

Second note: I notice your documents path is /home/deck/documents I don't have a steam deck in front of me to check, but my Linux system has a documents folder called /home/<me>/Documents with a capital D. Paths on Linux are always case-sensitive. That means /documents is not the same thing as /Documents, which is not the same as /DOCUMENTS/ and if you attempt to use one when it's actually the other, the file will not be found. Make sure the capitalization is correct in the whole path.

Step 2: Once you've located the correct path name of tailscale.sh you should be able to run it with: sudo <full-path-to-tailscale.sh>

Good luck.