He had removal orders. In 2019 an Immigration Judge ruled, due to gang activity in El Salvador, that he could not be deported to his home country.
The President of El Salvador has crushed the gangs in El Salvador since, so the Immigration Judge's determination regarding deportation to El Salvador may no longer be applicable. That issue hasn't been argued. Yet.
From SCOTUS .....
"The order properly requires the Government to “facilitate”
Abrego Garcia’s release from custody in El Salvador and to
ensure that his case is handled as it would have been had
he not been improperly sent to El Salvador. The intended
scope of the term “effectuate” in the District Court’s order
is, however, unclear, and may exceed the District Court’s
authority. The District Court should clarify its directive,
with due regard for the deference owed to the Executive
Branch in the conduct of foreign affairs. For its part, the
Government should be prepared to share what it can
concerning the steps it has taken and the prospect of further steps"
The "administrative error" was deporting him to his home country that no longer has the gang problem that it had when Abrego Garcia was denied asylum.
He had removal orders and the SCOTUS opinion is to facilitate his release and to send him somewhere other than El Salvador.