Salman said he would take military action to "eliminate this defense system."
Saudi Arabia and many of its allies severed ties with Qatar last year, based around a false media report claiming the Qatari Emir wasn't sufficiently hostile toward Iran. The Saudis have since suggested they'd channel out the entire Qatari border, turning it into an island, and dumping nuclear waste in the area.
Around this ever growing Saudi hostility toward Qatar, the nation made a deal to buy S-400 air defense systems from Russia. Russia says that plans to deliver the missiles have not changed, despite the Saudi threats.