you might not be able to make cleavage, but that doesnt mean you cant still enjoy the sensation of having your tits fucked. Even without cleavage to fuck, there's still something sensual in rubbing a penis on someone's breasts, against their nipples, slapping it against their sternum etc. It's still erotic, its still fun even if maybe it can't get someone off per se (or maybe it can! everyone's different!)
Contrary to popular belief, sex is more than just doing things with the express purpose of achieving orgasm. Sometimes its fun to do things that maybe cant make you cum, but are just like, aesthetically or emotionally pleasant. Making a sexual partner feel sexy and desired by ""fucking"" their flat or otherwise not-really-titfuckable chest has its own appeal and eroticism. Appreciating the feel of their skin and the touch of their flesh, and their weight upon you (or vice versa depending on the position). Teasing someone's cock with your nipples. Having someone jack off onto your tits! Fucking cleavage rules but its not the be all end all for making love to a person's tits.
also, if youre both open minded and like the idea of playing with toys or what have you, they do make big tiddy fake breast forms you can like, kinda wear like a shirt? Ive never tried fucking one but they look soft and jiggly and im sure it could be very fun in its own way. Try stuff out!

