"You are too late!"
了 is actually a difficult word to learn (and for me to explain).
In this particular sentence, the 太…了pattern is used to express a strong emotion (e.g. anger, disappointment) and thus it is not suitable to omit 了 here.
However, when 太 is used purely in the meaning of "Too" (without a strong modal expression) 了 is not always necessary. e.g. 对不起，因为你来得太晚，已经卖完了。/ Sorry, because you came too late, it has already been sold out.
I think what's going on here is the difference between arriving somewhere after you were expected too: 遲（迟）like if you're late for work, or the start of a meeting, and missing an opportunity you would have had if you'd been there earlier: 太晚了 like "I'm sorry, you're too late, someone already bought it."
I've never heard of using 太...了 with 遲, because 遲 already implies that some limit of lateness has been exceeded, whereas 晚 just means late with no implication that a particular time was expected.