I don't think that command will work. I'll discuss that after what you should be doing.
cd ~
ls -ltr
sh script.sh
The first command changes to your home directory. "~" is a shortcut to "/home/pi" which is user pi's home directory). I would recommend putting the script in your home directory, but if you want to put it in that download path, just change "~" to "/home/pi/Downloads/2017-11-29-raspbian-stretch-lite/".
The second command will list all files in that location. Assuming "script.sh" is there, move on to the next line.
The last line executes the script using the program "sh".
Your command:
/home/pi/Downloads/2017-11-29-raspbian-stretch-lite/sh script.sh
Breaking that down, you have the first part:
/home/pi/Downloads/2017-11-29-raspbian-stretch-lite/sh
That command runs the program "sh" from the "/home/pi/Downloads/2017-11-29-raspbian-stretch-lite/" path. If it's located there, you should be fine. However, the error you see tells you that Linux could not locate a file or directory named "sh" in that location.
Second, you are asking that program to use "script.sh" as a command line argument. "sh" will run a script that is on the command line, so as long as it can locate the file. I think the command that you should have run without any other information would have been:
sh /home/pi/Downloads/2017-11-29-raspbian-stretch-lite/script.sh
There's lots of ways to do anything, so this command or my other recommendation should both do the same thing. The difference is where the script executes from. If you are in a protected location (e.g., "/" which is the root of the file system), you may have errors when you download files via the script. In any case, give it a try and report back!