A snake slithers into a bar...

A snake slithers into a bar and asks for a beer. The bartender replies, "Sorry, we don't serve your kind here." "Why not?" asks the snake. "You can't hold your liquor."

This is another "walks into a bar joke". In this one, the critical element of the joke is a pun.

Snakes do not have hands (or any appendages for that matter), so they are not able to hold anything unless they are constrictors, but for the purposes of this joke we are ignoring constrictors I suppose.

Another sense of the phrase "hold your liquor" is to be able to drink without becoming obviously drunk (and presumably acting up), and it is possible that a bartender would deny someone service if they thought that they would be likely to act up after drinking.

So, the critical element is the idea that snakes cannot hold anything therefore they act badly after a few drinks.