"Li mouri anvanlè." oubyen "Li mouri anvanlè l"? Which one should I use?

You can use both.
anvanlè means prematurely or before one's time.

If you use the first one, then you are saying:
He/She died prematurely.
Li mouri anvanlè.

If you use the second one, then you are saying:
He/She died before his/her time.
Li mouri anvanlè l.

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words