It looks like it is a C19 to C20 cord. And no you need a proper outlet for it. https://www.infinitecables.com/products/iec-c19-to-iec-c20-power-cable-sjt Perhaps a list of the specs would help too, as the PSU on this likely to be quite a big one. What I see for your unit: https://www.delltechnologies.com/asset/en-us/products/workstations/technical-support/precision-7960-tower-spec-sheet.pdf Quite the possible range of configurations. More on reddit.com

In that case get the triple and sell 2, because that's a steal. More on reddit.com

Have you tried to reset the BIOS by remoing the CMOS Battery? Clearing the CMOS | Precision 7960 Tower Here is the BIOS Settings Documentation for the Precision 7960 Tower System Setup Options | Precision 7960 Tower Also the BIOS Settings Documentation mentions the following: (Everything from here has been copied from the above 2nd link for your ease). BIOS Setup > Boot Configuration > Secure Boot Feature: Enable Microsoft UEFI CA Description: Allows the UEFI CA to be added to BIOS UEFI Secure Boot database. When disabled, the UEFI CA is removed from the BIOS UEFI Secure Boot database. However, the Note section is more interesting NOTE: When disabled, the Microsoft UEFI CA could render your computer unable to boot, computer graphics may not function, some devices may not function properly, and the computer could become unrecoverable By default, the Enable Microsoft UEFI CA option is enabled. NOTE: This computer does not support Windows 11 Secured-Core PCs. Dell Technologies recommends keeping the Microsoft UEFI CA option Enabled to prevent graphics function loss and ensure the broadest compatibility with devices and operating systems. More on reddit.com

You cannot boot up installed Ubuntu or live cd? More on reddit.com