You can only bluff through emotes or chat words which can provoke your opponent and that is if you do it online, but it has no effect.
There are actually forms of bluff that can also be applied in online poker thru the actions at the deck or just even by the way a player is betting their chips. Not as effective as applying at the actual table but can still add some spices to the usual strategy when playing in online poker.
That's why you can see gamblers having a good run playing online poker. It means they are also doing some bluff actions aside from their already-knowledge skills.
The only way to bluff online is to avoid creating a game pattern.
For the rest, everything that is done has little value: phrases, smiles, etc...
By the way, many who in real poker were able to bluff online can be experts. But, there it is, they can only deceive novices. And even then it's hard, because even the novice can ignore these things.