I haven't used the randomForest package so I am working from general experience.
Shouldn't this line
pred.bagging<-predict(fit.bagging,data=(-xtrain))
be
pred.bagging <- predict(fit.bagging,data=(ytest))
That is, do the prediction with the data that you held back. Also, I would expect that the parameter for passing the data in predict() to be called newdata not data but I could easily be wrong about that.
In the line
MSE.bagging <- mean((ytest-pred.bagging)^2)
you are subtracting the pred.bagging values from an entire data.frame, ytest. Shouldn't you be referring to the predicted values, charges, from ytest?
MSE.bagging <- mean((ytest$charges-pred.bagging)^2)