You already have enough reputation to participate in chat. That's not what's going on there.
Since forever, chat rooms that aren't active get automatically frozen (read-only), then eventually possibly deleted (inaccessible). The criteria for deletion nearly perfectly matches the typical chat room that is created when comments are moved to chat, but the move-to-chat feature is younger than the delete-disused-rooms feature, and they have never been reconciled by the SE developers.
This results in getting “Page Not Found” when you go look for old chat rooms that were spawned from comments moved to chat, because they've been auto-deleted. These deleted rooms are technically still archived, but no longer publicly accessible. That's also for great reasons that make sense in other cases (a deleted room shouldn't be accessible, right?!), but doesn't quite make sense for moved-comments chat rooms.
There's a feature request on Meta.SO to have it changed so transcripts of such rooms are still readable — because wouldn't it make sense for those logs to be available still? — but it doesn't appear to have much traction: Make chat room transcripts forever public if the room was auto-deleted
Is there a workaround?
If a room is particularly important to humans despite not meeting the algorithm's sense of importance, it can be resurrected. Most aren't super-interesting, but your second example kind of has the intention of being longer-living: the idea was for it to be a place to discuss a certain topic, but as a room it never took off. (The actual activity in that chat room was almost non-existent.)
Most of the time, it won't be worth it, but in the rare cases where it is, a mod can resurrect a room by undeleting and unfreezing it; the easiest way being a flag on the associated moved-to-chat comment. If it garners sufficient activity after that, it will avoid auto-deletion again (though not freezing, but those are at least readable still). No promise that such requests will always be fulfilled, but then again, it's neither much work on our part nor much disruption if it was done without good reason, since it'll revert to the deleted state if it's not used.