1. The tavern is full of vampires and thus only "serves mortals" on platters or as mugs of blood to their regular clientele. As an added "plus" (more like a safeguard), it's also a non-alcoholic establishment.

2. The tavern is full of Dras and the barkeep is a Witch Doctor who only serves poisonous cocktails that would kill any other race. "We only serve plague here. Makes ya resistant!" (Or not.)

3. The tavern has a factional association or service "quirk" due to some local festival or tradition. You discover only those wearing certain colors on their headwear will be served. "Where's your colours?"

4. The tavern is cover for a kidnapping and slavery ring. As a person of standing or higher connections, you will be missed so they won't serve you their sleeping draughts.

5. You're a Waerd. The confused barkeep refuses to open one tab for everyone of your tribe who wanders in and claims to be you even before the last one leaves.

"This tavern is for Janoan warriors only, take your unblooded business elsewhere."

The tavern is in Dras lands and full of Dras. You're a Janoan (or vice versa). Fantastic racism at work

The tavern is a Hrothi one, and you're a Kypiq. The Hrothi bartender don't want to serve you because they're scared that you'll get drunk and go skittering through their tunnels.

You're a Dras who has removed their bracelets (look it up, it's a major faux pas in Drasean society).

You're a Janoa in kypiq lands, and due to how the two races are fundamentally opposed religiously, they do not trust you to not do something stupid or vicious while drunk.