Many thanks to HWTech for the valuable suggestions.
In following the suggested actions:
1. I uninstalled ClenMyMac from within the app as you kindly suggested and check for third-party apps. I have a few ones like, adobe, Skype, etc., deemed as Identified Developer, which I assume are allowed by Apple. I did not find any Unidentified Developer in System Information.
2. I started on safe mode and allow automatic installation of the upgrade which again failed to complete.
3. Still within Safe Mode I run Disk Utility on Macintosh HD (disk1s5) and Macintosh and this is a copy of the Show Details report. It appears that something is happening with the HD but I cannot understand the problem. Please help.
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Running First Aid on “Macintosh HD” (disk1s5)+
Verifying the startup volume will cause this computer to stop responding.
Verifying file system.
Volume could not be unmounted.
Using live mode.
Performing fsck_apfs -n -l -x /dev/rdisk1s5
Checking the container superblock.
Checking the EFI jumpstart record.
Checking the space manager.
Checking the space manager free queue trees.
Checking the object map.
Checking volume.
Checking the APFS volume superblock.
The volume Macintosh HD was formatted by diskmanagementd (1412.11.7) and last modified by apfs_kext (1412.141.2).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking snapshot 1 of 1 (com.apple.TimeMachine.2021-09-18-041203.local)
error: (oid 0xaf7d9) apfs_extentref: btn: dev_read_finish(718809, 1): Input/output error
Snapshot is invalid.
The volume /dev/rdisk1s5 could not be verified completely.
File system check exit code is 0.
Restoring the original state found as mounted.
Operation successful.
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Running First Aid on “Macintosh HD - Data” (disk1s1)
Verifying the startup volume will cause this computer to stop responding.
Verifying file system.
Volume could not be unmounted.
Using live mode.
Performing fsck_apfs -n -l -x /dev/rdisk1s1
Checking the container superblock.
Checking the EFI jumpstart record.
Checking the space manager.
Checking the space manager free queue trees.
Checking the object map.
Checking volume.
Checking the APFS volume superblock.
The volume Macintosh HD - Data was formatted by hfs_convert (945.200.129) and last modified by apfs_kext (1412.141.2).
Checking the object map.
Checking the snapshot metadata tree.
Checking the snapshot metadata.
Checking snapshot 1 of 1 (com.apple.TimeMachine.2021-09-18-041203.local)
error: (oid 0xaf7ca) apfs_extentref: btn: dev_read_finish(718794, 1): Input/output error
Snapshot is invalid.
The volume /dev/rdisk1s1 could not be verified completely.
File system check exit code is 0.
Restoring the original state found as mounted.
Operation successful.