Catalina software update issue

What current macOS are you running?

How is the device formatted? Catalina needs GUID/apfs

From Internet Recovery> Disk Utility>View>Show all Devices> your parent drive

here you can convert from HFS to APFS

Internet Recovery (option command R): https://support.apple.com/en-us/HT204904

arjundhas wrote:

Its running on OS X EL CAPITAN.

how to convert from HFS to APFS?

?

From Internet Recovery> Disk Utility>View>Show all Devices> your parent drive

here you can convert from HFS to APFS

Internet Recovery (option command R): https://support.apple.com/en-us/HT204904

Disk Utility>Edit>Convert to APFS

After that error does your computer boot normally in its previous operating system version or is the computer non-functional? It should have booted normally. If so, restart holding the R key and run Disk Utility. First run Disk First Aid to see if there are any unfixable errors. While you are there in DFA, see whether the drive is formatted APFS or HFS+ and let us know

arjundhas wrote:

Its running on OS X EL CAPITAN.

how to convert from HFS to APFS?

Did you run Disk First Aid and check drive integrity? The installer should do the conversion if the drive checks out ok. The way you worked the error made me think the drive needs to be repaired - which you'll have to do from either the Recovery Partition or Internet recovery.