There's no federal constitutional requirement for anyone to have legal training or certification to practice law in the USA.
The requirements to practice law in the federal system are set by the judiciary itself. This dates back to England where getting "called to the bar" meant the judge giving you permission to go to a physical bar separating the spectators from the court.
It wouldn't make sense to mandate judges to be lawyers if they decide who is and isn't a lawyer. That would give the judicial branch control over their own appointments.
