If from the first time he did not react to her, do not repeat yourself, and even more so, do not raise your voice. Just be quiet, take a break while waiting for the puppy's reaction. The pause should be short - 1-2 seconds, otherwise the puppy will completely forget what is expected of him. If he fulfilled, but the wrong command, do not praise him (when instead of "sitting" he lay down). Just force him to execute the desired command (sit down) and praise him.