When RubyMine reports Rubocop returning "exit code -1", upgrading Rubocop can be the fix:

gem install rubocop

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"The logs" can be accessed with Ctrl + Shift + A > Show log in (Files). This opens your file manager at the IDEA log location; the log file is called "idea.log".
If your operating system supports it, right click into the file manager > Open in Terminal. There run tail -f idea.log to follow the log.