It sounds like you own the Ercoupe from the Marketplace - is that correct? If so - you probably can’t use it yet. 3rd party 2020 marketplace planes haven’t all been moved to 2024 by Microsoft… yet.
In the new Marketplace tab (the one that’s just “coming soon”), there’s a “My Library” option at the top. That will show you all the add-ons currently available to you in 2024 (including anything in your 2024 Community folder). If the Ercoupe shows up there, you should be able to activate it. If it’s not there, you’ll unfortunately have to wait until Microsoft makes it available. When? Who knows? (I’m in that boat with the Transall.)
** I don’t think you can just move folders from the 2020 Official/OneStore folder into 2024. I’ve tried, and it didn’t work. (And in any event, it sounds like you want a cloud-streamed version, which that wouldn’t provide.)
If you bought it outside the marketplace, move the folder into the new 2024 Community folder. Then check the “My Library” tab - Community folder items aren’t always being activated when you first put them in. You may have to activate it from My Library and then restart the sim.
Hope this helps!