DIAGNOSIS That single-click sound is almost always a sign the chain brake is engaged. It's a safety lock that physically stops the chain from spinning when it's on.
FIX This is a simple one. Look at the front handguard, the big plastic shield in front of your left hand. Pull it back firmly towards the rear handle until you hear and feel it click into the disengaged position. You should now be able to pull the trigger and have the chain spin freely (without touching anything, of course).
IF THAT DIDN'T WORK If the handguard was already back and it's still just clicking, the battery might be too low. The saw needs a minimum voltage to run the motor, and a nearly-dead battery can sometimes still power the solenoid 'click' but not the motor itself. Try a different, fully charged DeWalt 20V Max battery.
HEADS UP Always check the chain brake before you start cutting. If you force the saw while it's engaged, you can damage the internal mechanism or the motor. It's designed to be engaged with a quick forward motion of your left hand if kickback happens, so getting familiar with its position is a key safety habit.