Acredito que aqui a maioria sabe que o Pop OS não usa grub, usa o systemd-boot. Eu encontrei esse tutorial aqui que explica certinho como instalar GRUB no PopOS e vou seguir ele para uma nova tentativa de instalação.
systemd-boot is only used with EFI and UEFi, so if you have the old BIOS grub should be installed by default. Your drive is /dev/sda and the partitions on the drive are numbered, so for example your drive D: is /dev/sda3. If you want now to (re-) install GRUB you are able to do this, with sudo grub-install /dev/sda because then you write GRUB in the MasterBootRecord of your Drive. In my last response I forgot that os prober is not installed by default. so you just have to install it with
sudo apt install os-prober, after the installation just run it with sudo os-prober and it should detect your Windows instalation. Then you open the grub config file. Personaly I prefer to do it with a console editor caled nano but you also could use gedit, what is more like the normal windows editor. The important thing is, that you open it as root, so you have to use sudo. So just run
sudo gedit /etc/default/grub. Then add the lines “GRUB_TIMEOUT_STYLE=menu” and “GRUB_TIMEOUT=-1”. If you have a line called “GRUB_HIDDEN_TIMEOUT” comment it by makeing a “#” on the beginning. Then save the file and close the editor and run
sudo update-grub. I think this should works and i answered the questions, if not just write.
Link de onde achei esse tutorial e que pode ser útil para demais pessoas.
É possível fazer essa instalação do GRUB em uma maquina com EFI/UEFI?