When I went downgraded my macbook 2011 to macOS El Capitan my Arduino will not open anymore. I had my macbook at High Sierra but I was having many troubles with that so thats why I can’t upgrade back to Sierra and High Sierra.
Here is the error message:
https://pastebin.com/ZSjeHGzQ
That is an Arduino issue. We have heard multiple reports; however, I am not sure if there is a permanent fix yet. I would check the Arduino forums… there seem to be numerous posts on the issue (feel free to “google” search it):
https://forum.arduino.cc/index.php?topic=354969.0
https://forum.arduino.cc/index.php?topic=399159.0