Crossvalidate (SVM)-classifier

Hi, I have the following code which runs fine as long as I have "crossval" set to "off". However, I would like to crossvalidate my data - but unfortunately then this piece of code throws an error - what exactly am I missing? Data files attached for reproducibility

kernel = 'linear'; C = 1;
cl = fitcsvm(datatrain,theclasstrain,'KernelFunction',kernel,...
    'BoxConstraint',C,'ClassNames',[-1,1],'Crossval','on');

[label,scores] = predict(cl,datatest);