Optiplex 745 MT BIOS 2.6.6 update payload error

Hello.

I got hold a of 745 MT and I have successfully updated to BIOS 2.6.4.

However, when I try to install 2.6.6 version I get the following message: 

I have run it as Administrator both under win 7 &10, tried to launch it via bootable MSDOS usb key but everything ends up the same way. 

Reason is I want to install a Q6600 inside will I be able to use it with BIOS 2.6.4?

Current config is C2D E6400/ 6 GB ram

Thank you in advance.