The number of DFRs you can make is determined by your Walkr Core level +1. So, if you are currently unable to build a DFR, it likely means your current number of DFRs has already reached its maximum. (For example: If your Walkr Core level is Level 10, you are able to build 11 DFRs). If you wish to build even more DFRs, please go to the Factory page and upgrade your Walkr Core level.