Most of that is false, and you should look at what media you've confused that has given you that false impression. In 2019, an immigration judge determined that Garcia was deportable because of his ties to MS-13 and issued a deportation order: https://www.supremecourt.gov/DocketPDF/24/24A949/354843/2025.... The relevant orders of the immigration judge and Board of Immigration Appeals are at appendix pages 1a-18a. The most relevant portion is 4a through 5a, where the Board of Immigration appeals affirms the immigration judge's determination that Garcia "is a verified member of MS-13."
> I'm also aware the Trump administration admitted they made a mistake in deporting him.
Incorrect. It is undisputed that, in 2019, the immigration judge issued a removal order. The only caveat was that Garcia could not be removed to El Salvador, his home country. From appendix page 46a (Garcia's brief in the current district court proceedings):
> Plaintiff Kilmar Armando Abrego Garcia ("Mr. Abrego Garcia") won an order from an immigration judge ("IJ") prohibiting his removal to El Salvador, after he established it was more likely than not that he would be persecuted in that country on account of a statutorily protected ground. The government could have chosen to appeal that order, but did not. The government could have chosen to remove Mr. Abrego Garcia to any other country on earth, but did not.