Neither plot works with your code. Try fcontour instead.
Otherwise, you have to vectorise your anonymous function for it to work without error:
f=@(x,y) x.^2+y.^2
The symfun version works without modification. If you want to use it as an anonymous function without having to vectorise it yourself, use the matlabFunction function:
syms f(x,y)
f=x^2+y^2-1
figure(1)
fimplicit(f, [-10 10],'bp')
f=matlabFunction(f)
figure(2)
fimplicit(f)
I added the ‘-1’ so it will actually plot something.
