Board avr-playground (Plattform avr, Paket Elektor) ist unbekannt

[SchneHa]

I installed both board packages (AVR boards and UNO R4) and tried to flash a sketch to AVR Playground. I got the error message that board avr-playground is unknown. I reinstalled the boards package several times. It's always the same.

How to fix that error?

Arduino IDE 1.8.5 for Mac

[per1234]

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:

1. 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.
2. Navigate to the packages/Elektor/hardware/avr folder.
3. 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!

[SchneHa]

I had several files of 0kb size in my Arduino15 folder. I deleted all of them and also deleted the folder of the previously installed board package. Then I downloaded the latest package folder from GitHub and copied it to the Arduino15 folder. Restarted Arduino IDE and all is fine.