The pushover analysis implemented in STAAD.Pro is based on the assumption that first mode of the structure is the fundamental (predominant) mode of vibration of the structure. Thus the analysis (implemented so far with this assumption) is valid for those buildings whose first mode has highest/higher mass participation in the lateral loading direction.
Also, as per Section 220.127.116.11.3 of FEMA356:2000 code (Chapter 3), lateral distribution of base shear is possible only when 75% of total mass participates for the fundamental mode in the specified direction of lateral load generation.
If you believe that your model satisfies this condition, you may try doing a pushover analysis. You can test it by asking the program to calculate the first 6 frequencies and viewing the ANL file to see what the participating factors are.
Also, the pushover analysis is part of the Advanced analysis engine, not the standard analysis engine. You need to ensure that you have the license for the advanced analysis engine.