Why does my structure say that it’s only designed for “monofacial”?

This is a mere wording. Whether a structure says that it’s designed for monofacial or bifacial modules in RatedPower doesn’t affect module compatibility as both structures can be used for any type of module. The only catch is that “bifacial” structures are simply claimed by the manufacturer to be specifically optimized for bifacial modules. Also, if you’re using a monofacial structure with bifacial modules, then the following characteristics will be applied for calculating the irradiance on the back-face of the modules:  

• Circular torque beam cross-section 
• 4 cm radius of the torque beam 
• 10 cm offset (measured from the back of the modules to the center of the torque beam). This results in a 6 cm space between the torque beam and the modules.