convert2rhel fails to register to Satellite with "HTTP error (422 - Unknown): Validation failed: Major version is not a number"

Issue

• convert2rhel fails with:

Attempt 3 of 3: Registering the system using subscription-manager ...
[08/21/2021 03:59:49] DEBUG - Calling command 'subscription-manager register --force --activationkey=key --org=Default_Organization'
HTTP error (422 - Unknown): Validation failed: Major version is not a number, Major version Operating System version is required
System registration failed with return code = 70
Trying again - provide username and password.
CRITICAL - Unable to register the system through subscription-manager.
[08/21/2021 04:01:20] DEBUG - Traceback (most recent call last):
  File /usr/lib/python2.7/site-packages/convert2rhel/main.py, line 86, in main
    pre_ponr_conversion()
  File /usr/lib/python2.7/site-packages/convert2rhel/main.py, line 179, in pre_ponr_conversion
    subscription.subscribe_system()
  File /usr/lib/python2.7/site-packages/convert2rhel/subscription.py, line 69, in subscribe_system
    register_system()
  File /usr/lib/python2.7/site-packages/convert2rhel/subscription.py, line 123, in register_system
    loggerinst.critical(Unable to register the system through subscription-manager.)
  File /usr/lib/python2.7/site-packages/convert2rhel/logger.py, line 104, in _critical
    sys.exit(msg)
SystemExit: Unable to register the system through subscription-manager.