I had this issue a few days ago as well, and it resolved itself after several restarts. It worked until the next restart, where it suddenly stopped working again. I find it very strange, because a print(error.debugDescription) seems to output "SFErrorDomain code = 1 (NoExtensionFound)". This error appears to indicate, that the App couldn't access the extension / the bundle id isn't found, what is very very strange, because the extension is clearly found in "Package Contents" > "Contents" > "PlugIns" and looking at my configuration, the bundle id's are matching. I have also checked "Allow Unsigned Extensions" in Safari. Any news on this? Thanks in advance! :)

Ok small update: Everything works now. I don't know why, I just restarted my Mac. I'm still confused. Maybe someone knows why on gods green earth that happened. (?)

Ah shoot, now I accidentally marked you answer as a solution... it wasn't... see I'm pretty new to this all. Uhm yeah.. tried your thing but it still showed me the same error code... Edit: can I mark the question as unsolved? This just annoys me haha Edit 2: I've created a new question with a bit more detail including the answer to the recommendation you made. Link - https://developer.apple.com/forums/thread/671024.