It looks like some entities in your model refer to a property that does not exist. This probably happened if you declared a property and had a mesh that was refering to that property. Afterwards you deleted the property.
If have to find out which elements refer to the missing property and just reaasign the property by using MODIFY-> UPDATE ELEMENTS -> PROPERTY ID
Try using View style -> Colour With -> Property Colours to figure out which elements correspond to which properties. before using this though, change the colour of each property in the property tree.
I've tried all that but still no luck. Other properties work fine just the plane strain ones. I even deleted the the whole model and started from scratch; defining the materials and properties.
This fatal error message may be specific to your used solver, which is NEi Nastran. Did you already discuss this issue with their helpdesk line? What did they say?
Are you running a specific group in your analysis? If so, is the missing property included in the group?