Biting side of playpen — how to tell puppy NO effectively?

Our 10-week-old golden retriever pup won't stop gnawing at the bars on her playpen. I don't really want her to be chewing on metal, and also in general I want to reinforce the idea that chewing her toys is okay, but other things are not.

I'm unsure how to communicate this to her, though. Every time she chews the playpen I say "No thank you!" kind of loud and move her away from the bars/edges. Usually I try to redirect her to another toy or chew (she has a ton of fun stuff in her playpen) but it's like no matter what I do she just keeps going back to the bars. It's kind of driving me nuts honestly.

Generally she really likes her toys and chews and is pretty good about playing with them on her own, but sometimes she just gets in these moods where she won't do anything other than go for the playpen bars. I feel like I'm spending my entire day going "No thank you!" and moving her away, ineffectively. I'm also a little worried she's starting to sense my annoyance and, as dumb as it sounds, I feel like we're both getting stressed over this back and forth all day long. And I don't even know if I'm communicating to her in a way she understands. A few times today I've resorted to giving her a timeout in her crate if she keeps doing it, but I'm guessing that's probably not the right thing to do either.

So how do I tell her "no" effectively? What do I do if redirecting doesn't work?

Obviously a yummy food treat or lick mat would trump the bars, but there comes a point where I can't give her any more food for the day.

Also just to note, she does get lots of exercise and supervised play during the day where she's out of her playpen, so it's not like she's cooped up in there all day. I'd just love to know how to get her to stick to appropriate chewing/playing during downtime after we're pretty much done with playtime for the day.

Thanks in advance 🙂