Thank you everyone for the advice but I took a different route and was successful!
After doing some research I got a few more articles on this issue and everywhere it was one or two package issues reported by user which was the culprit: e.g. python, gqrx-sdr, gnuradio, libomp5, pocl-opencl-icd, clang-6, etc.

So I reported this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal as mentioned in the error and it launched a browser window with that issue:

Taking idea from this and the above links, I read the 'main.log' file carefully and found that the following lines were giving a picture where the problem was due to which the upgrade was failing.

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2023-05-29 22:21:06,111 INFO installing python-is-python2 because python-minimal was installed
2023-05-29 22:21:06,111 DEBUG Installing 'python-is-python2' (python-minimal was installed on the system)
2023-05-29 22:21:06,141 INFO removing python-minimal because python-is-python2 is being installed
2023-05-29 22:21:06,141 DEBUG Removing 'python-minimal' (python-is-python2 is being installed on the system)
2023-05-29 22:21:06,141 INFO failed to remove python-minimal
2023-05-29 22:21:06,142 INFO checking for python-dev (auto_inst=True)
2023-05-29 22:21:06,142 INFO checking for libpython-dev (auto_inst=True)
2023-05-29 22:21:06,142 INFO checking for libpython-stdlib (auto_inst=True)
2023-05-29 22:21:06,142 INFO removing libpython-stdlib because None is being installed
2023-05-29 22:21:06,142 DEBUG Removing 'libpython-stdlib' (None is being installed on the system)
2023-05-29 22:21:06,142 INFO failed to remove libpython-stdlib
2023-05-29 22:21:06,142 INFO checking for libpython-dbg (auto_inst=True)
2023-05-29 22:27:33,918 ERROR Dist-upgrade failed
And 'python-minimal' and 'libpython-stdlib' were Broken in the 'apt.log'

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Broken libpython2-stdlib:amd64 Breaks on libpython-stdlib:amd64 < 2.7.15~rc1-1 @ii gK > (< 2.7.15-2)
Broken python2-minimal:amd64 Breaks on python-minimal:amd64 < 2.7.15~rc1-1 @ii gK > (< 2.7.15-2)
So I removed it:
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apt remove python-minimal
apt clean
apt autoclean
apt autoremove
Then ran the upgrade again
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 do-release-upgrade -p
And this time I didn't get an error related to broken packages
'apt.log' contained
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Log time: 2023-06-11 15:20:52.293358
Log time: 2023-06-11 15:21:03.127909
Log time: 2023-06-11 15:21:45.739526
'main.log' contained
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2023-06-11 15:21:41,293 ERROR IOError/SystemError in cache.update(): 'E:Release file for is not valid yet (invalid for another 2h 29min 25s). Updates for this repository will not be applied., E:Release file for is not valid yet (invalid for another 2h 30min 53s). Updates for this repository will not be applied.'. Retrying (currentRetry: 2)
2023-06-11 15:21:41,293 ERROR doUpdate() failed completely
2023-06-11 15:21:41,294 DEBUG abort called
So I followed the link I got after putting bug report command in the terminal and as advised I adjusted /etc/apt/sources.list file from bionic to focal

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# See for how to upgrade to
# newer versions of the distribution.
deb focal main restricted
# deb-src bionic main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to ## your rights to use the software. Also, please note that software in ## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb focal multiverse
# deb-src bionic multiverse
# deb-src bionic-updates multiverse
# deb-src bionic-security main restricted
# deb-src bionic-security universe
# deb-src bionic-security multiverse
deb focal-security multiverse restricted main
deb focal-updates multiverse restricted main
I ran the upgrade command again but got a different message (not an error anymore) which was promising

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baps@Aspire-A515-51G:~$ sudo do-release-upgrade -p
[sudo] password for baps: 
Checking for a new Ubuntu releasePlease install all available updates for your release before upgrading.
From there I followed another article

I followed Solution2 in there

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In this case, run the following commands to upgrade state cache.
$ apt clean 
$ apt autoclean Then execute the following commands.
$ sudo apt update 
$ sudo apt upgrade -y (this installed/upgraded almost 1600 packages so took a while. I did a reboot)
$ sudo apt dist-upgrade (this installed another 250 packages)
And voila! My Ubuntu is now upgraded to 20.04!!!I didn't had to run 'do-release-upgrade -p' command anymore or install the 5.4 Kernel, it was upgraded automatically with the above commands and old 4.15 Kernel was also removed

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baps@Aspire-A515-51G:~$ lsb_release -a
No LSB modules are available.
Distributor ID:    Ubuntu
Description:    Ubuntu 20.04 
LTSRelease:    20.04
Codename:    focal