She needed to prove herself to Jabba. This is a gangster, he doesn’t just let any schmuck in.
Would Vito Corleone just let anybody hang around his property?
Did Lando have to prove himself, though? Let’s say Lando did, and he brought in someone - great, he’s in. Now it’s her turn, but the point I’m trying to make clear is did she absolutely have to bring in Chewie? Clearly she could’ve brought in anyone else with a bounty if Lando was able to. Or, if Lando didn’t have to prove himself, and none of the other patrons do either, why should she have to? 😉