This error indicates something has gone wrong with a hardware package installed via Boards Manager. The solution is to remove and then re-install the packages. Be aware this will remove any boards you have installed via Boards Manager:
- (In the Arduino IDE) File > Preferences
- Click the link on the line following "More preferences can be edited directly in the file". This will open the Arduino15 (or similar name depending on OS) folder.
- Delete all files and folders under the Arduino15 folder except for preferences.txt. Please be very careful when deleting things from your computer. When in doubt, back up!
- (In the Arduino IDE) Tools > Board > Boards Manager
- Wait for the downloads to finish.
- Reinstall the packages you need (Adafruit SAMD Boards, Arduino SAMD Boards).
- Click "Close"