Remove the space in data 1
and data 2
. Also, the data are a list, not a chart:
data1 = With[{min = 0, fq = 5/4, md = 4, tq = 5, max = 9},
{min, fq, md, md, tq, max}];
data2 = With[{min = 2, fq = 3, md = 10, tq = 11, max = 18},
{min, fq, md, md, tq, max}];
BoxWhiskerChart[{data1, data2}]