Error On Running Ruby Code named "No SDKs Specified"

You need to configure the SDK here:

Use the Add SDK... button and specify the path to the ruby executable on your system.

On Windows you should install Ruby using http://rubyinstaller.org/ binary release. Not the source package.

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I fixed it like this (Idea 2019 Ultimate):

• File | Project structure | Modules
• select radio button on "Ruby SDK and Gems"
• Apply