How do I open the Beneviento mausoleum in the graveyard?

You must visit the mausoleum after defeating Donna but before progressing further. If you progress to the next area then the mausoleum never opens and you will be unable to retrieve the treasure.

You cannot get the stone slab before defeating Donna. The gate remains locked, and even if you walk all the way up to the treasure you cannot get back until you defeat Donna.

The crypt then won't open if you head towards the next boss where you can no longer return to the village until defeated.

FWIW, this page appears to back me up on this:

This treasure can be obtained AFTER you get the flask from Beneviento [...] This item is MISSABLE because the crypt closes later in the playthrough