If your espresso tastes sharply sour, lemon-juice unpleasant, or thin and aggressive, the shot is underextracted. Not enough of the coffee dissolved into the water during the brief contact in the portafilter, leaving the acids in the cup without the sweetness and body that balance them.
Three things to adjust, in order
Grind finer first. This is the most powerful lever. A finer grind slows the water down, increases contact time, and pulls more from the coffee. Adjust two clicks finer and pull again.
Increase your yield. If you are pulling 1:1.5 (18g in, 27g out), try 1:2 (18g in, 36g out). More water passing through the puck extracts more.
Check your shot time. Aim for 25-32 seconds from pump-on to your target weight. If your shot is finishing in 18-22 seconds, you are gushing through too fast regardless of grind setting.
Less common causes
Cold machine: pull a blank shot to flush the group head and warm everything up before your real shot. Old beans: anything past four weeks from roast date is fighting you. Fresh-roasted beans (3-21 days post-roast) extract much more easily.
What sour is not
Sour is not the same as bright or fruity. A well-extracted Ethiopian washed coffee will taste of citrus and stone fruit. That is acidity working with the rest of the cup. Sour is when the acid stands alone with no sweetness behind it.