Plot the Load Proportionality Factor (LPF) and you should see a change in the loading at the point where the structure has buckled. You can then calculate the buckling load by knowing the LPF and the magnitude of your actual loading.
Check in the manual; "Unstable Collapse & Post-Buckling Analysis".
I checked the LPF but up to very large values for LPF the graph is still straight. what do you think may be the problem that it doesn't actually buckle