English Translation
Hawaii is like a paradise on Earth.
Why isn’t it “auf der Erde”? Is it a fixed expression?
Pretty much.
See here: auf Erden - Wiktionary, the free dictionary
Another fixed expression would be “die Hölle auf Erden”.
“auf Erden” by itself is “chiefly literary or religious”, as wiktionary says, but “ein Paradies auf Erden” and “die Hölle auf Erden” are fixed expressions that can still be used in modern german.