It looks like Mint configured your system to load its secure boot keys into your computer, but didn’t place the files necessary to do so on the boot drive.

If this is an installer drive, I’m very confused why Mint would try to load MOK Manager before installation. I can’t explain the problem in that case, so I wouldn’t know how to fix it. Perhaps a different installer image (an older one?) might help there.

If you’ve installed Mint onto the flash drive, something went wrong setting up your bootloader. If your computer supports browsing to an EFI file to boot, you should try finding /EFI/BOOT/grub2/grub.efi (or something named similarly) to boot once. From there, you should find instructions for Mint to reinstall your bootloader. Hopefully it’ll be configured correctly once the bootloader is reinstalled.

Another problem that may occur is that your motherboard is reordering drives between insertion. Your computer is currently configured to load EFI/BOOT/mmx64.efi from a specific EFI partition, but if your motherboard found another EFI partition (like, for example, one for a Windows install), it’ll get confused and fail to boot from the right place. If that’s the case, there’s not much you can do to fix the problem except for updating your motherboard’s firmware and keeping your fingers crossed. A workaround would be to copy the missing files and directories from your flash drive’s boot partition to your computer’s, making sure not to overwrite any files. This may require redoing after bootloader updates, but it would get the system booting again.