[Solved] Help! “Failed to open \EFI\BOOT\mmx64.efi

For those having the same problem I manage to overcome it by doing this: I putted the Linux Live USB on my other computer, went to the ¨\EFI\BOOT\¨ folder inside the flash drive and copied the ¨grubx64.efi¨ file to my desktop, renamed it to ¨mmx64.efi¨ and putted it back on ¨\EFI\BOOT\¨. After doing that I went back to my Dell notebook and booted it through UEFI USB, I now got the following error: ¨error: shim_lock protocol not found.¨. I then proceeded to my BIOS and disabled Secure Boot, and booted it through UEFI USB again. Now it worked, I proceeded to the normal Linux Mint installation and everything installed as usual. After turning on the notebook again, and seeing that Linux was correctly running at its internal drive, I turned off the notebook, went into its BIOS again and turned Secure Boot back on. After rebooting it, it finally prompt me the Enroll MOK blue screen, and I could properly enroll the MOK with the password I choose on the first installation (the one that started all this mess), and it finally resolved this problem definitely. The BIOS or NVRAM or whatever was no longer waiting for this MOK to be enrolled again. Now if I ever reinstall Linux Mint in UEFI mode with Secure Boot on, this error would not show up again, unless I did what I did the first time I caused this problem, which was to give up the installation after the screen that asks me if I want to install the multimedia codecs or not and to choose a password to enroll the MOK.

I think this is an issue with the Linux installation that should be reviewed, maybe the installer shouldn't ask for the NVRAM to enroll the MOK until after the installation process is complete, at the reboot part.

And Reddog1, not every newer notebook supports legacy mode anymore, each is the case of my brand new Dell notebook. Also I wasn't with dual boot with Windows, the notebook came with Ubuntu, I just formatted it and put Linux Mint on it. Funnily enough the notebook came with Secure Boot on, even if being a Linux laptop. The fault of the error was entirely on the installer side, but I will follow your advice and always turn off Secure Boot before installing Linux on any machine.