I am trying to flatten a cylindrical/circular surface, can you help; my flatten option is grayed out on Solidworks 2017. If it's posible for you can you create one so I can use as sample?
You are not providing us much info. Flattening is a sheet metal process. You have to convert to sheet metal. If you have a tube, you have to rip it to create a seam.
There is a Surface Flatten option in 2017. If you haven't already read the help. If your still having trouble using it, then post more information so we can help you better.
That is not a surface, that is part with Mass. Attached is how you flatten a surface.
Since you do not have flat face, you cannot use convert sheet metal on your part. My suggestion, is that you try offsetting a surface @ 0.0, and using the Flatten surface command like I did in the attached file. (Version 2017)