Here's why:
* It lacks a complete thought: While it tells us something about Daniel, it doesn't convey a full idea. We need more information, like what he can come to or when.
* It's missing a subject and a verb: The phrase has a verb ("can come"), but it's missing the subject (who is doing the coming).
To make it a complete sentence, you could add:
* Daniel can come to the party.
* Daniel can come tomorrow.
* Can Daniel come? (This is a question, but still a complete sentence.)