When you burned the .iso file to your flash USB, did you also
verify the ISO image (SHA256 Checksum) prior burning it ? If those characters don't totally match up, I know a Linux install can go awry like that. And that your image software may not necessarily detect a problem when you make the flash drive.
See the link below pertaining to "integrity & authenticity":
https://www.linuxmint.com/edition.php?id=312
You might also consider downloading it again from another server source, presumably one that your browser will tell you if it's safe to do so. The last download mirror I used safely was "kernel.org".
Otherwise I take it that your BIOS is set up with UEFI and Secure Boot? With version 21.3 it should crank out that grub file without incident. Made to run alongside Windows, though dual booting may prove precarious as
@Outdated pointed out above. Microsoft doesn't want to play fair.