BRUSSELS—EU antitrust enforcers have claimed a court made legal errors when it scrapped their order for iPhone maker Apple to pay 13 billion euros ($15.7 billion) in Irish back taxes, in a filing to have the verdict overturned. The stakes are high for the European Commission in its crackdown against what it sees as aggressive tax planning by multinationals. It has a mixed record to date, winning court backing in its case against Fiat Chrysler but losing in the Starbucks and Belgian tax break cases. The Commission is appealing to the Luxembourg-based Court of Justice of the European Union following a ruling last year by the General Court, which said the EU executive had not met the requisite legal standard to show Apple had enjoyed an unfair advantage. In its 2016 finding, the Commission claimed two Irish tax rulings had artificially reduced Apple’s tax burden for over two decades, which …