As Hyejeong mentions, -예요 is frequently misspelled as -에요, even by many native speakers. To make matters complicated, however, one cannot always dismiss 어디에요 as wrong because there are cases where it is legitimately used. Here are examples of both phrases. A: 지금 어디예요? Where are you now? (예요 is an informal-polite form of 이다) B: 지금 은행이에요. I'm at the bank now. AA: 어디 가세요? where are you going? BB: 은행에 가는 길이에요: I'm on my way to the bank. (AA didn't catch "the bank" part) AA: 어디에요? To where? (에 here is location particle, and 요 a politeness suffix) The last line is constructed from 어디에 (matching 은행에) followed by 요, not 어디 + 에요. So 어디에요 is correct and 어디예요 is not in this context. All this is rather esoteric for new learners, so if you were not asking about such minute detail, you can just disregard this. In spoken language, the two phrases are not easily distinguishable anyway, so people will not even notice it if you say the wrong one. It is mostly important in writing and in understanding the underlying principles.

It is kind of funny that I never thought “어디에요” is actually not correct. I am a native Korean speaker and I seriously never really see people saying “어디예요?” but “어디에요?” I myself has been using 에요 instead of 예요 bc we never pronounce 예요 “yeyo” but “eyo”

어디예요 is the right spelling. Koreans often misspell -예요 as -에요 because the letter 예 can be pronounced as 에 in combination with CONSONANTS (계, 폐, 혜, 례, etc.) After vowels 예 should only be pronounced 예 but many Koreans overextend the rule, confusing people who are learning the language.