It is important to note that many bosses cannot be rebuked after you've parried them once, because one parry won't be enough to break their stance. Most of the time, you have to parry the boss several times before you are able to land the riposte.
What are the attacks you can parry in Elden Ring?
While you are able to parry many physical attacks in Elden Ring, you can't take on all of them. Most attacks coming from enemies that are gigantic in size can't be fought, and attacks from enemies with weapons of large size with two hands can't be defeated either. It's also not possible for you to parry bolts, arrows, or shield bashing attacks. Also, you cannot combat attacks from beasts.
When it comes to what you can parry, assaults that come from "regular" weapons as well as large guns that are one-handed can be dealt with. Alongside standard attacks, you can parry various Ash of War weapon arts that involve physical attacks.