This error can be caused by something going wrong during a Boards Manager installation. That can often be fixed by manually removing and then re-installing the boards platforms via Boards Manager:
Be aware this will remove any boards you have installed via Boards Manager. They are easy enough to reinstall again though.
Delete all files and folders under the /home/jjc/.arduino15 folder except for the preferences.txt file. As noted in your other thread, by default the folders that start with . are hidden by default, so don't be deterred if you can't see the folder at first glance. Please be very careful when deleting things from your computer. When in doubt, back up!
Now you will need to re-install the boards you just uninstalled:
- Select Tools > Board > Boards Manager from the Arduino IDE's menus.
- Wait for the update to finish.
- Reinstall the boards platforms you need.
- Click the Close button.
I use Windows for my primary operating system for the same reason. I would switch to Linux otherwise.