I have an Nexus 6 Version 7.1.1. The complete Storage is crypted. So it is not possible to wipe the data and system by using TWRP. I did an factory reset but after that the storage also was crypted. How can I encrypt these Version of Nexus 6?
I believe that the stock rom forces encryption.
Use TWRP's format data to remove encryption if you want to install a recent custom rom. Of course, you'll lose all your files by doing that.