People Sit on Chairs
Mouse: ...what does that mean?
Switch: It doesn't mean anything.
Tropes are conventions used in storytelling to convey some sort of information to the audience. People Sit On Chairs don't convey any meaning—they aren't storytelling conventions at all, they're just things that happen normally or incidentally during the storytelling. So if somebody is calling "Chairs" or "PSOC" on your trope proposal, this means they think your idea is about as meaningful as the discovery that various different shows portray people sitting on chairs: It doesn't matter how commonly it occurs, this is something that never carried any meaning to begin with, making it Not a Trope.
The whole point of a chair, after all, is to give people something to sit on, but this by itself doesn't convey meaning. On the other hand, there are ways in which chairs can be used for a trope; perhaps we're talking about an impossibly awesome-looking chair (or a more functional Super Wheelchair); maybe it gets used for a Chair Reveal, or for the Big Bad to express his Slouch of Villainy. And maybe the chair is conspicuous by its emptiness. All these add some sort of meaning to the chair in question, which makes them legitimate tropes.
Note that the criticism here isn't simply that the trope in question is "too common" or "too broad", as No Trope Is Too Common. There are some extremely common cliches, and Omnipresent Tropes, that appear in fiction maybe even as frequently as chairs, but these are still storytelling devices. For instance there is The Couch, another item people sit on, but given a purpose that correlates with the visual layout of house floorplans. Conversely, a trope suggestion can still be guilty of People Sit On Chairs even if it doesn't literally appear all the time. Even if it is relatively rare, it can still be used without a narrative purpose.
If you are really, really, really sure that we need this one, try to connect it to something meaningful. If not, you just need to accept that this trope page will never happen. If you don't know whether your trope proposal is a case of People Sitting On Chairs, try asking "Is This Tropable?"
On the other hand, if your trope is something like Chairs Sitting on People, then you have the opposite problem: Too Rare to Trope, where the example you provide is unique to the point of being extremely and memorably notable, yet impossible to extract trends from.
See also Consistency for the other aspect of defining a trope.
We could list examples here, but that would be as futile as cataloguing all the blades of grass on the planet Earth.