how can I solve this error in this code?

S=tf('s');
G1=1/(S+1)^4;
G=G1/S;
Gr=opt_app(G1,0,1,1);

Unrecognized function or variable 'opt_app'.

k=Gr.num{1}(2)/Gr.den{1}(2);
L=Gr.ioDelay;
T=1/Gr.den{1}(2);
[Gc1,Kp1,Ti1,Td1]=folipd(1,K,L,T,10);
[Gc2,Kp2,Ti2,Td2]=folipd(2,K,L,T,10);
step(feedback(G*Gc1,1),feedback(G*Gc2,1))

[Mohamed Khattab]

@Jan@Walter Roberson

Answer 1

[Walter Roberson]

opt_apt appears to be part of the File Exchange Contribution https://www.mathworks.com/matlabcentral/fileexchange/60874-fotf-toolbox