Why I am getting a Warning Rank Deficient message?

I have a somewhat simple code where I am filling in a zeros matrix of nX3 in order to solve the problem A*x=b for x. My code is as follows:

A1 = zeros(n,3); X = zeros(n,1); for j = 1:n; A1(j,[1,2,3]) = [-(St + j), St + pf*(j-1), pc*(j+1)]; end B1 = [ P(n+1), P(n), P(n+2) ]'; %P(n+1)==P(n), P(n)==P(n-1), P(n+2)==P(n+1) B1 = A1\X

where St, pf, pc are constant values.

I am prompted with the following message:

Warning: Rank deficient, rank = 2, tol = 4.490849e-14. > In SSBinFisModel at 17

What does it mean, and how can I resolve it to get an answer? Thank you!