Currently, it is not possible to drink milk from the backpack in the Fabric version of the mod. (Wiki) #1274 #1200 #1133 #415

Milk has been removed from fabric version, if you find any other mod that adds milk as fluid (placeable in the world or something) I can add compatibility

Originally posted by @Tiviacz1337 in #1133

Milk lib adds support for milk as a fluid, and it is already compatible with the tank upgrade for the backpack (can be sucked/placed in), you just cannot drink it. As far as I can tell this is not a standalone mod, but can be included in other mods. It is supported by Tech Reborn, Create, and AE2. I think Create is the one that actually "includes" it, but I can't tell even after looking through their source code. The creator also has a "Milk Plus" mod that includes it, but that mod is not in the modpack I am using.

Technical Details
Modpack: Prominence II Hasturian Era 3.1.53hf2
Minecraft 1.20.1
Fabric 0.16.14
Traveler's Backpack 9.1.37

Integration will be included in the next update

Integration added in 9.1.39
https://www.curseforge.com/minecraft/mc-mods/travelers-backpack-fabric