Do not know if Boot-Repair, grub, UEFI or just modprobe is wrong. Boot-Repair & grub seem to run modprobe, but it wants efivars in a kernel subdirectory. Really mounted here, sys/firmware:
efivarfs on /sys/firmware/efi/efivars type efivarfs (rw,nosuid,nodev,noexec,relatime)
Some systems have another UEFI setting beside UEFI Secure Boot, for locking install of boot loaders, booting USB drives or other settings. Check you manual & UEFI settings.
May be similar. Forum closed, but still read only, If you set UEFI password, never lose it, or reset to blank when done. Otherwise very difficult to repair.
Fujitsu lifebook ah512. Secure boot options blocked in bios - UEFI password required
You can only install one nVidia driver. You get conflicts as another install, does not cleanly remove everything from previous driver. Best to always purge & install correct driver from Ubuntu repository. If from nVidia directly as .run file, you its procedure to uninstall. You need to be booted into terminal or from grub’s recovery mode, or chroot. Often then easier to reinstall if nothing changed, and choose to install optional drivers.
sudo apt-get remove --purge nvidia-*
sudo ubuntu-drivers devices
sudo ubuntu-drivers autoinstall
Nothing you do in Ubuntu should damage Windows. But good backups always the number one recommendation for both Windows & Ubuntu. And have both Windows repair/recovery flash drive & Ubuntu installer flash drive for repairs. But if using device like /dev/sda or /device/sda1 always double check that it is the device you expect. Drives are loaded in UEFI/BIOS order. My flash drive as sdc, becomes sda on reboot. Your Windows drive may become sda or NVMe drive as first device.
Most systems auto find Windows in UEFI. And you should always be able to boot Windows from UEFI as sometimes you may need minor fixes. Grub only boots working Windows or Windows that has fast startup off and does not need chkdsk. Windows is known to turn fast startup on with updates and may update UEFI firmware resetting UEFI to defaults. Remember any UEFI settings & Windows settings you change. My Dell needed no UEFI settings & older desktop needed 7 settings in UEFI redone. Dell has Windows and needed fast startup & bitlocker off.