RMSE, R square question calculation

i have my confusion matrix as C.mat

8263    20    39
2       3826  14
43       7    4431

My predicted class labels are Ypred and actual labels are Ytest. Ypred size is 16000*1 and Ytest 16000*1

I am trying to calculate the R sqaure and RMSE and I defined the following:

   rmse = sqrt(immse(Ypred, Ytest));

I also tried to use the built in function to generate R SQUARE between two vectors

   mdl=fitlm(Ypred,Ytest,'linear')

But both didnt work. Is there something I am missing?