Bert wants to go shopping but has lost his keys.
Oh, the breakdown in communication. Bert gets mad that Ernie won't let him in, and Ernie gets annoyed (at least a little bit) by Bert's insistent "practice".
I think it's fair to say that Bert is hoist by his own petard here, in that if he hadn't mentioned wanting to practice, then Ernie would have probably let him back in right away. On the other hand, Ernie is going to be sorry when he gets hungry for supper and there's no food.
Also, why didn't Bert just say, "I forgot my money and I can't buy food without it, so please let me back in"? (Because there wouldn't be a sketch otherwise, of course.)
Susan and Gordon have apparently installed a doorbell since the last time someone was out in the hall.