I downloaded ubuntu 20.04 and started installation for dual boot with Windows 10 OS.
While checking disks, it indicates file error ie error in the disk.
Installation bootable pendrive is OK.
I continued.
Deleted old ext4 partition and swap partition.
Created ext4 partition and swap partition from the same free space.
After some time it says: can not copy files, disk error etc. Tried many times.
Installation terminated.
I installed back Ubuntu 18.04.4 in a similar manner. It installed successfully.
As I advised in the comment, you should verify the checksum of the downloaded Ubuntu 20.04 ISO. On the same URL from where you downloaded Ubuntu 20.04, you should also have the checksum number.
You can run the checksum on the ISO file you downloaded and compare it with the checksum number on Ubuntu’s website.