"The cat drinks its milk."
Translation:De kat drinkt zijn melk.
Well, in this context you can't use ervan, it's not a possessive. Ervan means 'from it, of it' ( https://www.vandale.nl/gratis-woordenboek/nederlands-engels/vertaling/Ervan#.W4CljKgaU0M ).
The only way of using ervan would be if you had already mentioned the milk and so don't want to repeat yourself:
Er is daar een kom melk. De kat drinkt ervan.