While you can do both (and should strive to do so), when you're prioritizing what to focus on, you'll have to pick one to put effort into. So which is more important to convey: good gameplay or good story?
You can (and probably should) do both at different stages of the product. How much or how little you invest into each option is entirely up to you.
You should probably spend more time on whichever of the two interests you more. The most important thing is to be able to finish your game, and you'll have a better chance of doing so if you are engaged with and enjoy the product you're making. No matter how much you like the process, there will be days when you're quite sick of it, so anything you can do to stack that deck in your favor helps.
You can have gameplay without a story (c.f., Tetris). But while you can tell story without gameplay, that's not a game. Even so-called "walking simulators" (like Firewatch) have a small element of mechanical interaction within them. Consequently, I'd generally advise that in the earliest stage of a project you focus most on the gameplay. Hone at least the core loop of game until it feels fun.
After that, if you have a story in mind, you can switch gears to focus more on its pacing and its integration with the mechanics of the game. Part of that process will often involve wanting to tweak mechanics to better mesh with the story -- that interplay is a unique feature of a game, after all, and it's good to play it up.
But as above, cycle between these in proportion to how they fit into the kind of game you're making, which in turn should reflect at least in part the kinds of things you enjoy in games yourself.