I have seen  these types of errors when the iso download is corrupted. Please try verifying the hash of the file using the appropriate hash checker for your OS (see image below). if that fails, you might want to try just updating the idrac first: https://www.dell.com/support/home/en-us/drivers/driversdetails?driverid=p8hc9&oscode=wst14&productcode=poweredge-r740xd

#Iwork4Dell

Check sums are ok

Thanks

I just downloaded the iso and ran it on my R740xd, no problems. Luckily I even have the same version of bios as you. my idrac is at 4.20.20.20. If you dont mind losing any bios settings you have set, you could try setting the bios to defaults using the default button in the lower right corner of the F2 (bios) screen. 

#Iwork4Dell

I am hesitant to apply default bios settings

I will try using Lifecycle controller and use downloads.dell.com

Thanks

OK thanks

It works fine with Lifecycle Controller