"I don't think I've learned how to properly call functions because I keep getting errors when trying to run function."
The problem is how you have written the function: you defined one input argument RADIUS which you did not use anywhere. And then inside the function you refer to a variable/function named R which you did not define anywhere.
You also need to use array operations for element-wise operations:
r=randi(10,3,1);
Area = AreaSphere(r)
function A = AreaSphere(radius)
A = 4*pi.*radius.^2;
end % ^^^^^^^^
