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For just trying things out, you don't need an arnold license; it can just run watermarked. If you're worried about errors, then in the ArnoldGlobalSettings node, under Error Settings please make sure abort_on_license_fail is unchecked, and then it should still render, but with watermarks.
For a trial license, etc you'll need to see https://www.arnoldrenderer.com/arnold/try/ for more info.