said that a Mexican-American judge could not be fair in ruling on the border wall?

Trump made the claim that because Judge Gonzalo Curiel is Mexican-American, he was unfair in his past rulings against Trump University, and had a conflict of interest in his upcoming decision on the legality of the border wall. The Indiana-born judge ultimately ruled in favor of the wall’s legality.

The Wall Street Journal – Trump Says Judge’s Mexican Heritage Presents ‘Absolute Conflict’
The Hill – Mexican-American judge who Trump attacked rules in favor of border wall

Politifact – Donald Trump’s racial comments about Hispanic judge in Trump University case