DIAGNOSIS That's the mower's safety system telling you it's not in a ready-to-run state. The most likely cause is you're not holding down the safety key/lever on the handle while you pull the start lever.
FIX You have to do both actions at once. With the battery installed and charged, firmly grip the handle so your palm is depressing the safety lever. While holding that down, pull the start lever back toward you with your other hand. You should hear a single beep and the blade will engage. If it beeps three times, you're not holding the safety lever down fully or you're trying to pull the start lever first.
IF THAT DIDN'T WORK Check that the battery is fully seated and locked into place. A slightly loose connection can cause the controller to freak out and throw the safety beeps. Remove the battery, wipe the contacts with a dry cloth, and push it back in until it clicks.
HEADS UP This mower won't start if the bag is full or the rear flap is open. The three-beep error covers a few 'not ready' conditions, so make sure the grass bag is properly attached and the deck is clear of debris.