I have dual booted(win 7 + ubuntu 14.04) last year. But few days ago I can't boot to any OS, so I decided to install new Ubuntu 18.04 only. So, when I made a partition table and proceed to install, this lines are prompted. I'm not quite familiar with these.
The partition table format in use on your disks normally requires you to create a separate partition for boot loader code. This partition should be marked for use as “EFI boot partition” and should be at least 35 MB in size. Note that this is not the same as a partition mounted on /boot.
If you do not go back to the partitioning menu and correct this error, boot loader installation may fail later, although it may still be possible to install the boot loader to a partition.