Unable to start the editor.

I've installed the editor and the game file on both the same drive. However, when launching the editor, I get the following errors:

Inspecting more closely, it seems that the "sbox-dev.exe" is in the sbox game folder but steam is looking for a completely different folder that is empty.

Happened to me. I have to manually add "sbox-dev.exe" to steam to get my shortcut..

 I think it’s a temporary problem; you can solve it by creating a symlink between sbox 2129370 (editor) and sbox (game). 

My case is exactly the same. I am currently living in mainland China, and even after installing the file, the files won't download and it shows the exact same error message.

Hi
I saw this workaround in sbox discord

1) Close steam

2) go to your \Steam\steamapps folder

3) edit appmanifest_2129370.acf and change installdir to "sbox"

Note: Make sure you installed SBox BEFORE sbox-editor

My computer's appmanifest_2129370.acf has "installdir" set to "sbox".
But when I try to install the sbox editor through Steam, it shows the download size as 0B. Is this because I'm in China?

No it's perfectly normal
You need to install and start at least once S&Box then install S&box editor (which is just a shortcut to sbox-dev.exe, that's why it's 0B)

And if it still state "missing exe" follow the workaround

It still state "missing exe"  and i can't find "sbox-dev.exe" at the folder (I instailled it to D drive of my computer)
The Korean text of the error message below means 'Missing game executable'
and i found "sbox-dev.exe"at other folder named "sbox590830" 

Funny
then instead of just putting "sbox"  in the  appmanifest_2129370  try to put "sbox 590830" (not sure about the whitespace too)

Edit: I think I know why, you probably installed sbox-editor **before** s&box 

I found a solution for that:

https://sbox.game/f/devhelp/1849/1/#post2

Do NOT install the editor. It seems like it is meant for the official release down the line, later in April. Follow my steps in the post I linked - this resolved all editor issues for me (I had several...). Hope it helps.