She can not only write but also read.
In this sentence, can we use "can't"?

Yes, you can, but "can not" is more common. It also enables emphasis to be put on "not". "Cannot" is also possible.

PaulM

"She can not only write but also read."

Sigh! Why not simplify the whole thing and say: "She can write as well as read."

Because that's not what was asked. It was about a construction that happened to contain the coordinating element "not only", which is frequently used to emphasize the content of the second coordinate.

In future, read the question properly. And I don't think that in general cutting across perfectly good previous answers is helpful.

PaulM

You're really too pedantic. When a questioner makes an awkward observation, simplification makes communication clear and precise. Also, sir, just because your answer may have been a good one, does not mean it is the ONLY one. No one on this site is the final authority, neither you nor I; we are merely contributors.

Excuse me. It has nothing to do with being pedantic, but answering the question that was actually asked. Of course English often has several different ways of saying the same or similar things, but this was not an instance of the questioner asking for an alternative, but simply asking whether "can't" may be used instead of "can not" in the example given.

Note also that can't or can not is the title of this thread.

PaulM