Error upgrading from SLES 12 SP5 to SLES 15 SP3

Dear SUSE Community,

Greetings of the day!!!

I am doing an upgrade of VM from SLES 12 SP3 to SLES 15 SP3. I have successfully upgraded to SLES 12 SP5 through online only.

From 12 SP5 to 15 SP3, as per my analysis it will be done through offline only am i correct?

For this Process, I have downloaded ISO image and attached hardware to the KVM. From boot menu, I have selected attached image.

After mounting step, We are getting below error:

Error message:
Internal error: Failed to open TCP connection to (Network is unreachable - connect(2) for “” port 443)

There is no blockage in host machine also. I am able to access suse website in host machine.

Could you please help me on this?

I was strucked from last three days. Please help me on this.

Much appreciated.

For more error info, Please check the attachment.