Arduino IDE won't upload sketches
You may be able to fix this by taking some drastic action. Be aware this is somewhat advanced and will remove any boards you have installed via Boards Manager:


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.
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!
Restart the Arduino IDE.
Verify you can now upload before re-installing any boards via Boards Manager.