I'm installing Ubuntu 18.04 LTS on a Dell Inspiron 5559 with 1 TB HDD where WIndows 10 is already installed, hence it's a dual boot. I have set up a Ubuntu bootable USB drive and booted Ubuntu in UEFI mode from it. I have disabled secure boot, fast boot and created suitable partitions on Windows already.
The installation starts smoothly but fails again and again with the message "unable to install GRUB on dev/sda1" and says that it is a "fatal error". /dev/sda1 is the EFI boot partition (ESP partition) on my HDD to which Windows Boot Manager is installed. I had read that GRUB bootloader must be installed to that partition.
I am somewhat certain (but not completely) that the Ubuntu OS installed perfectly and the problem cropped up only in the GRUB installation, since whenever I try to re-install, the setup wizard says that "this computer has Windows 10 and Ubuntu 18.04 LTS on it" and gives the option to "Erase Ubuntu 18.04 LTS and re-install", which would only be possible if Ubuntu was already installed.
I have tried boot-repair, which says that that there was an error during repair due to which it could not be completed. Pastebin : http://paste.ubuntu.com/p/J3dk2YGBqG/
I'm a complete newbie at Linux/Ubuntu and would be grateful if anyone can provide a solution to this problem.