function F = Main(x) ↑ Error: Function definition not supported in this context. Create functions in code file.

function F = Main(x)
    F(1) = x(1)^2 - 2 * x(2)^1 - 1;
    F(2) = -3 * x(1)^2 + x(2)^2 + 2;
    F(3) = x(1)^3 + x(2) ^ 3 - 2;
    [x,fval] = fsolve(Main,[0, 0 , 0],options)
    options=optimset('Display','iter')
    
    

Answer 1

[Zhonghua Sun]

From the above, it seems that the function of fsolve() is not defined before referenced.

Answer 2

[Walter Roberson]

function F = Main(x)
    F(1) = x(1)^2 - 2 * x(2)^1 - 1;
    F(2) = -3 * x(1)^2 + x(2)^2 + 2;
    F(3) = x(1)^3 + x(2) ^ 3 - 2;

You need to store the above 4 lines in a file named Main.m

    [x,fval] = fsolve(Main,[0, 0 , 0],options)
    options=optimset('Display','iter')

You need to store the code

    options = optimset('Display','iter');
    [x,fval] = fsolve(Main,[0, 0 , 0],options)

in a different file, not named Main.m . Then you would execute that file.