You need to use the griddata function, not interp2:
x=[0 2 4 6 8];
y=[0 2 4 6 8];
T=[100 90 80 70 60; 85 64.49 53.50 48.15 50; 70 48.90 38.43 35.03 40; 55 38.78 30.39 27.07 30; 40 35 30 25 20];
x_target=4;
y_target=3.2;
T_target=griddata(x,y,T,x_target,y_target)
T_target =
44.4580
figure(1)
surfc(x, y, T);
hold on
stem3(x_target, y_target, T_target, '^r', 'MarkerSize',10, 'MarkerFaceColor','r')
hold off
grid on
view([-130 25])
The interp2 function requires that your data be gridded before using the function. The griddata function creates the grid itself.