Is it only your warlock and titan that can't access it? Both of those characters have yet to complete a rumble match, it should unlock after you do though.
Edit: saw that you mentioned your hunter could already, just complete rumble on Titan/warlock and they'll be good to go too
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Thank you! I'll try that out! I can't believe I never played rumble on my Titan lol thanks a ton
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Haha np once you do it'll become selectable