Alone time barking often starts because leaving is a big event. Keys, shoes, and long goodbyes can also show up as puppy barking at the door: https://lovemypuppy.store/barking-puppy-training/
Step out for ten seconds and come back calmly. Grow the time only when your dog stays loose. A safe chew can help some dogs. Panic signs mean you should slow down and get skilled help. Pick up... https://lovemypuppy.store/barking-puppy-training/