If all the coffee shops in your area serve unbearably sour espresso, there might be 2 possibilities: either all of them suck and can’t properly extract a light roast, or you might be hypersensitive to acidity. Personally, I love acidity and I enjoy fruity, pleasantly acidic espresso as much as sweet, chocolatey shots, but I get that for a lot of people, “coffee” tastes burnt and bitter.

Coffee is naturally acidic. Very acidic. Acid compounds are also among the fastest to extract. The lighter the roast, the harder it is to extract most of the coffee, therefore light roasts tend to be more acidic than more heavily roasted beans. Light roasts are all the rage ATM because they respect the beans’ origin and characteristics and highlight rather than hide the specificity of high quality beans. The more you roast, the more you lose the original character of the beans and get burnt flavors (this is why most commercial coffee is heavily roasted, the more you burn it, the more you hide the flaws of you cheap, commodity shitty beans).

Espresso extraction follows a curve :

• First drips: salty and very thick
• under-extraction: sour and thick
• sweet-spot: sweet, chocolaty and syrupy
• over-extraction: bitter, watery.

Since roast levels affect the ease of extraction, the same ratio on a light roast and a dark roast will be wildly different. a 2:1 ratio for a light roast would be still way under-extracted, therefore a longer ratio should be used to get some sweetness to compensate for sourness, and get a slightly acidic shot (2.5:1 to 3:1 ratio should be fine). Dark roasts will overextract much quicker and get bitter much sooner, and therefore should be shorter to be palatable. A medium roast should be in the Goldilocks zone at a 2:1 or slightly longer ratio.

TL;DR: ask for a longer shot or a medium roast if you want less acidity and more sweetness and chocolatey flavors, a traditional dark roast blend with up to 20% robusta if you enjoy old school bitter espresso, or drown it in dairy, sugar and spices.