Lomb Scargle periodogram calculation

Hey.. I have to find periodicity using lomb scargle periodogram. I tried with the following function:

https://in.mathworks.com/matlabcentral/fileexchange/20004-lomb--lomb-scargle--periodogram

But I have few doubts like: how to calculate the error with the calculated period value? IN the comments section its saying to calculate FAP as: with FAP of ',num2str(prob(jmax)) BUt its giving a very very less error as compared to the period calculated.

PLease help me with the above issue.

Thanks