Sorry if this is a dumb response!
The pictures show four wheel nuts, one with a key slot (the locking wheel nut).
This is a "dummy" key slot!
No key goes in there.
This is actually a cover that pulls off the actual locking wheel nut. There is a special tool that makes this easy (or less difficult!).
Once this is off then the locking nut is behind it, and you should have a special wheel nut tool (like a socket) that engages with it and enables it to be undone.
If you have lost this socket, then follow the advice in postings above.
peter