Rank deficient warning when performing secant method

I am trying to find an a that returns the value from roret(n,a) to 120. this is my code so far. How can I make it stop when I get an a that gives me roret(n,a) = 120?

I get this following error: Error in roret (line 26)

A(n+1,n-1:n+1) = [(k/(2*h)) (-4*k/(2*h)) ((3*k)/(2*h))+a]; %last row in matrix

Even though when I run roret(n,a) seperatly it works fine.

Please help!

format long

n = 200;

a0 = 20;
a1 = 21;

iter = 0;
tol = 1e-90;

d0 = roret(n,a0);
while abs(a0-a1)>tol   % Secant method
    d1 = roret(n,a1);
    a2 = a1 - ((d1*(a1-a0))/(d1-d0));
    d0 = d1;
    a0 = a1;
    a1 = a2;
    iter = iter +1;
    d2 = roret(n,a0);
    if d2 == 120
        break
    end
    
end
a = a0

[Pravin Jagtap]

Hello Mhamed,

To understand what's going wrong in the user-defined function 'roret', we need access to it. Share the implementation of 'roret' so that we can help.