"Tap to start!"
Tap what to start? Game doesn't even work. Pressing left or right arrow makes a click sound. Clicking with mouse buttons anywhere does literally nothing.
--edit--
After reading what @HerbieG wrote, I played around with it a bit. Maybe what he wrote works for some. The only thing that worked for me was fullscreening the game, then pressing F5 to reload the game *again* once it was already fullscreened. Regardless, this sort of extreme problem is absolutely unacceptable in any game and is an exceptionally *good* reason to give it a low score or even a score of 0, contrary to what he said.
This is such a trivial thing to get right, and so game-breaking if you don't, that there really isn't any good excuse for it. Go read some JavaScript tutorials or something if you can't even get something as basic as capturing input of mouse clicks down. Or if you used some other tool to make this, go read up the documentation for that. Zero excuse for this kind of brokenness!
Even had it worked correctly from the start it's still not a great game and nothing that hasn't been done better elsewhere. Although there's plenty of examples of other very awful flappy games, at least most of the others actually work from the start!