unsupported Java Version

Please see https://intellij-support.jetbrains.com/hc/en-us/articles/206544879. It looks like you override the default JetBrains Runtime either via the environment variable or via idea.jdk file in the config directory (https://www.jetbrains.com/help/idea/tuning-the-ide.html#config-directory).

my current Intellij version

For IntelliJ IDEA CE the config directory would be /Users/<user>/Library/Application Support/JetBrains/IdeaIC2020.3

If you have idea.jdk file there, delete it.

Thanks Serge! After delete the idea.jdk file, it works now.

I'm on a PC, serached the whole AppData\Roaming folder and don't see idea.jdk. I'd love to find and delete that and experience excellent results.

Brian

On Windows it's either idea.jdk file or environment variable, please see if you have any set: https://intellij-support.jetbrains.com/hc/en-us/articles/206544879.

Brian

it should be in the following path: Users\<user>\AppData\Roaming\JetBrains\IntelliJIdea2020.3

Thanks Serge, same worked for me with PHPStorm; deleting /Users/<user>/Library/Application Support/JetBrains/PhpStorm2020.3/phpstorm.jdk

the new java (version "15.0.1" 2020-10-20)
does not understand 
-XX:+UseConcMarkSweepGC

specified on Linux in

~/.config/JetBrains/IntelliJIdea2020.3/idea64.vmoptions

needed to remove -XX:+UseConcMarkSweepGC from the idea64.vmoptions file

For PyCharm I needed to remove /Users/<user>/Library/Application\ Support/JetBrains/PyCharm2020.3/pycharm.jdk