tried installing using 'something else'
allocated 1GB for ESP, 100GB for root, 100GB for home and 50GB for swap area
then received a message saying 'attempt to mount a file system with tyoe vfat in SCSI1 (0,0,0), partition #1(sda) at /boot/efi failed.
ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.15.10
ProcVersionSignature: Ubuntu 5.8.0-43.49~20.04.1-generic 5.8.18
Uname: Linux 5.8.0-43-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.445.1
CurrentDesktop: ubuntu:GNOME
Date: Wed Jun 16 13:03:14 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
ProcEnviron:
LANGUAGE=en_IN
PATH=(custom, no user)
XDG_RUNTIME_DIR=<set>
LANG=en_IN
LC_NUMERIC=C.UTF-8
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)
According to the logs, sda1 is formatted as an ext2 partition; so it's not an ESP since an ESP must be FAT.
It's possible we could improve ubiquity's handling of this case, but if you are skipping ubiquity's support for automatically configuring your partitions correctly, you're somewhat on your own for getting the partitions right.