Input type error symbolic tool kit

I just installed symbolic tool kit and not familiar with it.

I am using Steepest Descent Method to solve a system. Following is my code:

syms x1 x2 x3
f =@(x1,x2,x3) (x1^3+x1^2*x2-x1*x3+6)^2+(exp(x1)+exp(x2)-x3)^2+(x2^2-2*x1*x3-4)^2;

syms phi(t)
x0 = [1 1 1];
change = 1;
xi = x0;
g = gradient(f,[x1 x2 x3]);
kk=1;
eps=1e-05;
while change > eps
    
    gt = double(subs(g, [x1,x2,x3], xi));
    
    if gt == 0
        break;
    end
    
    tt = xi - t*gt';
    phi = (subs(f, [x1,x2,x3], tt));
    phi = symfun(phi, t);
    phi(t) = simplify(phi);
    phi_dash = diff(phi);
    ti = solve(phi_dash, t);
    ti = double(ti);
    im = imag(ti);
    re = real(ti);
    ri = im./re;
    ti(ri>1e-3) = [];
    ti = real(ti);
    temp = double(phi(ti));
    [~, te] = min(abs(temp));
    ti = ti(te);
    
    xt = xi;
    xi = xi - ti*gt';
    
    change = (xi-xt)*(xi-xt)';
    error(kk)=change;
    kk=kk+1;
end
kk=1:kk-1;
plot(kk,error)
xlabel('Iteration')
ylabel('Error')
title('Plot showing Error convergence')
disp(strrep(['Solution is: [' num2str(xi, ' %0.2d') ']'], ',)', ')'))

It runs on r2016a with lots of warnings but I am using r2019a at home and I am getting the following error

Error using subs
Expected input number 1, S, to be one of these types:

sym

Instead its type was function_handle.

Error in sym/subs (line 60)
validateattributes(F, {'sym'}, {}, 'subs', 'S', 1);

Error in (line 20)
    phi = (subs(f, [x1,x2,x3], tt));

I am downloading r2016a now but it's really slow and I'd like to debug this to run on r2019a.

Please help!