After beating Nitros Oxide, as at the start, you’ll get a cutscene. He will state that you need to do some stuff and then you can challenge him again. To be able to challenge him, you must collect everything - relics, CTR Tokens, Gems (unlocked from Gem Cups, unlocked with CTR Tokens), and anything else. I’m pretty sure that you can get different ranks of Relics - this doesn’t matter, just unlock the Sapphire Relics (lowest rank) and you’ll be a-ok.