He can, the special counsel isn’t protected by anything really. All of the recent special counsels (like Mueller too), essentially serve at the pleasure of the Attorney General.
As part of the fallout of Watergate, legislation was passed that allowed for the appointment of “Independent Counsel”, an Independent Counsel could be requested by the House or Senate Judiciary Committees, but it was still up to the Attorney General to make the decision on whether or not to appoint one. Once a decision was made to appoint an Independent Counsel, a special three judge panel from the Courts of Appeals made the decision on which attorney to hire. The Independent Counsel could not be summarily dismissed, only specific wrongdoing or a few other things could justify early termination. However, the law allowing for Independent Counsels had a time limiter, it was extended a few times, but was allowed to expire in 1999–probably at least in part because the final Independent Counsel, Ken Starr, had attracted tremendous negative publicity due to perceptions of wasting time and money on the Clinton investigations.
Ever since then it’s just been “special counsels”, that AGs appoint (generally under political duress), they typically leave them alone. However, John Durham wasn’t even originally appointed as a special counsel, he was doing this investigation under orders of Bill Barr, for purely partisan reasons, as a U.S. Attorney. Near the end of Trump’s presidency he resigned as a U.S. Attorney but was appointed by Barr as a special counsel, specifically to continue his political investigation in a way Democrats would be loathe to dismiss.
I meant it when I said Durham had a respectable legal career, and for that alone I would not have been in favor of Garland summarily firing him day one, but now that it’s gone on for years, and all he has to show for it is a failed prosecution on a terribly stupid charge that almost certainly was never going to succeed, I sadly have to say Durham’s competence and integrity are in question as is the legitimacy of his behavior. That plus the unusual and partisan nature behind his appointment suggests to me Garland would be entirely correct to fire him, legally, ethically, politically et cetera.