Why must DNA have thymine instead of Uracil?

Uracil is readily produced by the chemical degradation of cytosine so have thymine as the normal base makes detection and repair of appearing mutations more efficient

Uracil is energetically less expensive to produce than thymine, which may account for its use in RNA. In DNA, however, uracil is readily produced by chemical degradation of cytosine, so having thymine as the normal base makes detection and repair of such incipient mutations more efficient.