AR Spectrum Estimate with Covariance Method Order Problem

Hi all. I have data matrix y(80x200). I want to use AR Spectrum Estimate using Covariance Method (pcov) for orders 10,50,100. But I'm getting error at order 100. What should I do to fix it? (like zero-padding, maybe?) Or am I never be able to use order 100?

 x=y;         %y(80x200)
 order=100;   %10,50,100
 nfft=80;
 pxx = pcov(x,order,nfft);