Polypropylene has material properties not unlike other plastics or metals. You have a linear range to work in up to the yield point. So, nailing down what the yield point is would be important for you. Wikipedia has a value range (12-43 MPa) for polypropylene that is a wide ballpark to use for now (Ultimate tensile strength - Wikipedia) while you secure more accurate data from your material supplier. Judging by the plot you have showing only 260 kPa you are probably well under the lower end of that range (12 MPa). Although, your mesh appears to be very coarse judging by the blotchy coloring. You would want to reduce the mesh element size from here and observe how the stresses change until the results don't change as you reduce the mesh size. I don't think you will jump from 260 kPA to over 12 MPa but it doesn't look like you have converged on your stress value yet.
I wouldn't say you necessarily need SOLIDWORKS Plastics or nonlinear analysis to go further on this (at least not from what you are showing here).