I'm trying to install Windows Server on my machine which also has Ubuntu GNU/Linux, and Windows 10 Enterprise installed on it.

When I load my installation media...
(notes about installation media:)
Created with Rufus
ISO downloaded directly from Microsoft servers (no pajeet torrents)
Checksum verified

I see a screen which says the following:

A recent hardware or software change might have installed a file that is signed incorrectly or damaged, or that might be malicious software from an unknown source.

If you have a Windows installation disc, insert the disc and then restart your computer. Click "Repair your computer," and then choose a recovery tool.

Otherwise, to restart Windows so you can investigate further, press the ENTER key to display the boot menu, press F8 for Advanced Boot Options, and select Last Known Good. If you understand why the digital signature cannot be verified and want to restart windows without this file, temporarily disable driver signature enforcement.

File: \Windows\system32\winload.exe

Status: 0xc0000428

Info: Windows cannot verify the digital signature for this file.

Now then, I'm not sure how Windows 10 (Enteprise) was installed, but I'm aware that Ubuntu GNU/Linux was installed via BIOS mode. I'm not very well-versed in EFI, but I understand how to configure a BIOS.

I don't know why the message is requesting media -- I'm booting from an installation disk.