Solving nonlinear equation, what does the message shown mean?

I have to solve for d minimzing norm of Y for some initial value

I have data for all other variables other than d

I tried to use fminunc, lsqnonlin, fsolve functions but I am getting the message as

"lsqnonlin stopped because the final change in the sum of squares relative to its initial value is less than the default value of the function tolerance."

eta=4/3;
R=28.5;
delC2p=1.4063;
delC2=2.3344;
delC1p=1.1250;
delC1=1.6594;
tL=atan(28.5/6);
tC=atan(28.5/12);

psi=((eta^2*d*sin(tL-tC)/(delC1p*cos(tL)))^(2/3));
Y=(R*(sqrt((psi-eta^2)/(eta^2-1))-tan(tC)))/(tan(tC)*(sin(tL)/(sqrt(eta^2-sin(tL)))-sqrt((psi-eta^2)/((eta^2)*psi))))-d;

Answer 1

[Matt J]

It means that LSQNONLIN thinks that it has found a solution. It has managed to reduce the squared error to less than some fraction of its initial value and considers this a reason to stop iterating.