If you only have one drive then I would recommend using the automatic partitioning options of the installer. Something like “erase disk and install ubuntu”, but I can’t remember the exact wording.
This should install the bootloader correctly. If it doesn’t it will report an error message and installation will not be complete.
If you subsequently mess things up then you can repair grub2 using the instructions in this post Repair Grub 2 from a live DVD . If you are using a removable drive then you can use
sudo grub-install --removable