https://downloads.arduino.cc/packages/package_index.json file signature verification failed. File ignored
Try this:In the Arduino IDE, 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 IDEAfter that, try your Boards Manager installation again.
Are you really running OS.X 10.4 (Tiger), or was this a typo for 10.14?Min spec for Arduino 1.8.9 is OS.X 10.8 (Mountain Lion)
Are you behind a corporate firewall? Someone had this same problem when their firewall was injecting a warning into the downloaded file.