They already "accidentally" deported a citizen in Maryland and are arguing they can't get him back because he is in El Salvador where they have no jurisdiction. It is a test to see if they can get away with it.
He (Kilmar Abrego Garcia) is not a citizen, nor did he enter the US legally. He entered the US illegally as a teenager but was granted temporary protected status because of a credible fear of gang violence if he returned home[1]. A US citizen cannot legally be deported[2] unless they are first denaturalized (which is extremely rare and has not occurred under Trump).
First they would have to convince the Supreme Court that birthright citizenship isn't in the Constitution, which seems like a long shot even for this court.
[1] https://www.bbc.com/news/articles/c62gnzzeg34o
[2] https://en.wikipedia.org/wiki/Deportation_of_Americans_from_...