"I am looking for a large building."

Translation: Én egy nagy épületet keresek.

[AtalinaDove]

When are you allowed to omit "egy"?

[RyagonIV]

Although I have to add that in many cases it sounds much better with egy. Like here.

• "Egy nagy épületet keresek." - "I'm looking for a (certain) large building."
• "Nagy épületet keresek." - "Just show me any large building you can think of."

[linguafiqari]

Always, except when counting, of course.

[AtalinaDove]

Ah. So I can report this sentence for not accepting it without?

[linguafiqari]

Yep

[MrtonPolgr]

This is simply wrong and it just leads to a lot of frustration and confusion. I can foresee people asking "why was it not accepted without egy" under several exercizes. And natives will simply tell you that "it doesn't sound right without egy".
So far, I have seen like one person trying to approach this topic apart from me which isn't a lot, so take this as a rule of thumb rather than something academic. Not using any articles is a bit like uncountable nouns in English, except you can do it with basically any noun: 1. it's very indefinite about the amount (as well), it's left to the context 2. therefore it doesn't introduce a new instance, you are much less likely to refer back to something you only mentioned without "egy"