Calamares oem user setup removes /boot/amd-ucode.img

[Wollie]

Try to solve it in analogues procedure:

Deleted intel-ucode

The system should still be bootable if you edit your Manjaro boot entry in grub (press e in grub menu) and adjust the initrd line from initrd /boot/intel-ucode.img /boot/initramfs-5.4-x86_64.img to initrd /boot/initramfs-5.4-x86_64.img and then press f10 to boot this edited entry. Reinstall the package or at least update-grub to get a bootable system.