Hi
note not a teacher nor a native speaker
He: You mustn't park there.
That sentence can be said for necessity or compulsion in the future as well as in the present.
I would put it into reported speech :
He said that I wasn't allowed to park there. (present)
or
He said that I'm not allowed to park there. (future)
for both meaning could be used :
He said that I mustn't park there.
On occasion it can be ambiguous,
Cheers,