I need to write a sentence, and I have problems with whether I need to use "an" or not.

I use the text of books to search for similar sentences, since i'm a beginner and I have problems with grammar.

The sentence, which I need to write: Surgeons can plan (an) operation.

The problem is that I don't know if I should necessarily use "an"? I have found in books both a variant with "an" and without. And that makes it difficult for me to understand.