So it seems @jyu found the problem with his particular board-file installation. This info and the other tips given above might help to find the issue with your own installation, maybe too long or non-ASCII filenames, YMMV.
If all else fails, we advise to follow @Luc’s idea as a fallback. There was a similar issue mentioned elsewhere on the forum and if you scroll down on that page you’ll see our detailed instructions to copy the board files manually:
Downloading missing Board binary - stuck - #15 by MrBlueSea
(SquareLine Studio looks for the ‘Arduino’ folder in its folder created in ‘Documents’, and the subfolders containing the board-templates, to build its list of ‘Create’ targets at program-startup.)