x is a vector so x <-1 is also going to be a vector of logicals. In your case neither of your if statements returns true so y is undefined. You need something vectorised like the following (or a for loop if you want to be deliberately inefficient!):
x=-10:0.01:10;
y = zeros( size( x ) );
y( x < -1 ) = 2 * x( x < -1 ) + 1;
y( x>= -1 ) = 2.^x( x >= -1 ) + 1;
In your code x<-1 would only return true if every element in the vector x satisfied that condition, which is obviously not the case.
