Welcome to the forum! 
As @stephane said, apt is not officially supported. Manjaro’s package manager is pacman, or alternatively, pamac. apt as found in the AUR is an experimental port of Debian’s package manager to Arch and Arch derivatives. Do not use it in Manjaro.
That said, the error you’re getting requires a reinstallation of GRUB. Boot up from the Manjaro install medium in live mode, and open up a terminal window. Then issue the following commands… 
sudo su -
manjaro-chroot -a
grub-install --recheck --no-rs-codes --modules="part_msdos part_gpt zstd"
update-grub
exit
It should now be safe to reboot.