Watch the video:
Timestamps:
0:00 - Explanation
0:35 - Disk failure?
0:58 - Repair existing Steam Library folders
1:20 - Fix can't create new Steam Library
2:13 - Method 2: Manually add the Steam Library folder
Want to install or update games but can’t? Want to install games to another drive in Steam? Well, if you’re getting the “Steam Library must be writeable” error, this video will show you how to fix it! By the end, you should be able to download, install, update and play the game as usual, hopefully.