Negative value for saturation mode voltage of MOSFET
If \$ V_{GS} \$ is smaller than your than your threshold, \$V_{tn}\$, then that means your FET is in cutoff region, meaning that your drain current \$I_D=0\$ and your FET will not conduct. Here's a good table from the Sedra\Smith textbook that gives a good summary of the regions of operations for the NMOS device, where \$V_{ov}\$ is the overdrive voltage.