That's interesting that actually many Vietnamese people cannot drink milk because they doesn't tolerate a lactase enzyme.
Why is this not ok to translate to "I drink glass of milk"?
Because the translation is intended to be in normal / proper English.
In normal English, we would say either, "I drink THE glass of milk," or, "I drink A glass of milk," but not "I drink [ ] glass of milk."
The glass of milk...
The or a is missing in our choices.