Errol Garner couldn't read a note... but he happened to have perfect pitch, which means that you hear a note and can instantly name it (C, Eb, Bb, etc) or even correctly identify all the notes within a chord. Pretty useful when you are trying to learn a song and don't have a music sheet or maybe want to transcribe a solo.
I haven't had such a luck and so, when I started playing the guitar (around 15 years), spent hours with my ear glued to the gramophone to understand what the guy was playing exactly; I even slowed down the speed from 33 to 16 rpm, but of course the pitch went down as well...
Today we have transcriptions and all kind of pitch-shifting devices, but I still get frustrated when I am playing a song and then try a little improvisation but soon run out of ideas or my fingers refuse to play what I hear in my brain, and then I regret not having had a proper education in music, so I could at least play all those scales up and down the keyboard.
But here again technology helps: I can record a track at a time, correct my mistakes redoing small bits instead of the whole song and finally come to a result that leaves me somehow satisfied.
The most difficult part has been stopping regretting not to be a better player and instead accepting my own limits: within those limits music has given me a lot of pleasure and, when I am deep immersed in the arranging process, often come up with musical ideas that really surprise myself.
I consider this a gift from the Heavens and can only guess what kind of elation must give composing a whole song, especially one of those timeless classics that we still enjoy to play.