Why does Red Hat Enterprise Linux 6 to 7 in-place upgrade fail with "ValueError: kernel name must start with '/boot/vmlinuz-'"

Issue

• Trying to upgrade Red Hat Enterprise Linux 6 to Red Hat Enterprise Linux 7 using in-place upgrade procedure.

• redhat-upgrade-tool command fails with

  # redhat-upgrade-tool --iso /tmp/rhel-server-7.6-x86_64-dvd.iso
  Traceback (most recent call last):
   File "/usr/bin/redhat-upgrade-tool", line 571, in <module>
     main(args)
   File "/usr/bin/redhat-upgrade-tool", line 405, in main
     upgrade_boot_args()
   File "/usr/lib/python2.6/site-packages/redhat_upgrade_tool/boot.py", line 169, in upgrade_boot_args
     '--update', kernelver(kernel)])
   File "/usr/lib/python2.6/site-packages/redhat_upgrade_tool/boot.py", line 32, in kernelver
     raise ValueError("kernel name must start with '%s'" % kernelprefix)
  ValueError: kernel name must start with '/boot/vmlinuz-'