Why is it wrong to use "on je" here? On je vysoká osoba? On je vysoký člověk?
because that would be "He is a tall person", not "It is a tall person"
Why can't you say Je to osoba vysoka?
Adjectives are placed in front of the noun they refer to.
what is osoba?
Why isn't it "Je TA vysoka osoba"?
It is this "Je to...", "To je..." special and very common type of sentence. You will see it a lot. It is always TO, even for plural.