Being inexperienced in big tournaments like this one might be a problem for Graham Potter maybe. They couldn't beat Salzburg for example but it was his first Champions League game at the same time. Chelsea still have four more games to play here. Nothing has ended for them yet but on the other hand they are going to play against Milan two times in a row now. This is the most critical time period of the group stage for them. If they don't get at least four points from these games then their chance to move on can be much lower depending on the results of Dinamo Zagreb - Salzburg games.
Chelsea have actually made the transfer they need for solving their scoring issues. But let's see how good Aubameyang will be able to perform. If he doesn't perform like he was at Barcelona then they might still have serious issues.
Inexperience during champions league shouldn't be a problem because their opponents are not that much amazing aside from maybe Milan, so they should be second at worst no matter what. The fact that they are not shows that it is not just inexperience in UCL, but inexperience with a big club like this before.
Potter has been a manager for a while now, but he hasn't really managed a team at the size of Chelsea before, and definitely never been with a club that spent 200+ million in a single off-season. That type of size definitely requires a bit of experience, which you gain by doing it for the first time, because nobody grows knowing it, it just so happens that Potters first time will be with Chelsea.