Windows Phone Recovery Tool 2.1.1 bug "cant verify the package "

[Jayc7x]

I wanted to roll back to win 8.1 after downloading 1.7 Gb of data it says "unable to verify package " Phone Model Lumia 730.
Pls help me i was able to roll back to denim from version 1.2.4 but this new version seems to still create problem.