The "Board ... is unknown" error occurs when a Boards Manager installation of a different version of a hardware package fails to completely uninstall the previous version. Here's how you solve it:
- Click the link on the line following File > Preferences > More preferences can be edited directly in the file. This will open the Arduino15 (or similar name depending on OS) folder.
- Navigate to the packages/esp8266/hardware/esp8266 folder.
- You will see two folders. One is named by the version of the package you currently have installed, the other is named by the version of the package you previously had installed. Delete the folder with the previous version name. Please be very careful when deleting things on your computer. When in doubt back up!
After doing that, the error should be fixed.
FYI, in the Arduino world, the correct term is "upload", not "download".