Edit the file /etc/default/grub and change the following line… 
GRUB_TIMEOUT_STYLE=hidden
… into… 
GRUB_TIMEOUT_STYLE=menu
Also make sure you add the following line to the file before saving… 
GRUB_DISABLE_OS_PROBER=false
Save the file and then run the following command in a terminal window… 
sudo update-grub