Brewing is coffee grinds dissolving. Acids leave the grounds first, then sugars, and bitter compounds at the end. If your brewed cup tastes sour, water left the grounds before it was able to get to the sugars. That is an under extracted coffee, the most commonly found fault in all home brewed coffee, and by a wide margin too.
The fix for this is the grind size. The grind size is the biggest variable you have control over. A finer grind does two things: it creates more surface area for the water to work on, and slows down drainage so that the water during the brewing process spends a longer amount of time in contact with the grinds. Both of these things increase extraction. If you’re going up in gears on your grinder at home, change slowly. Small moves to perfect your cup. Jumping multiple settings up will most likely push your coffee to be over extracted and not taste like anything.
If the finer grind does not help solve the issue, the next part of the brewing process to address is your water! Specifically, the temperature of your water, which most people don’t believe. The working range is around 93 degrees Celsius, or about 199 degrees Fahrenheit (a couple degrees higher or lower is okay). Water cooler than that just does not have the energy to extract from the grounds properly1.
Brew time is the last thing to check, and this works hand in hand with the grind size itself. And last is the ratio of water to coffee: if you are using more water than coffee, the grounds do not have enough contact, and too much water results in a diluted cup.