Description of problem:
'The service-level command is not supported by the server.' when list available service-level with Credentials on a unregistered RHEL9.0

Version-Release number of selected component (if applicable):
candlepin-4.0.5-1.el7sat.noarch
subscription-manager: 1.29.15-1.el9

How reproducible:
always

Steps to Reproduce:
1. List available service-level by 'subscription-manager syspurpose' on a unregistered RHEL9.0:

[root@kvm-07-guest01 ~]# subscription-manager identity
This system is not yet registered. Try 'subscription-manager register --help' for more information.

[root@kvm-07-guest01 ~]# subscription-manager syspurpose service-level --list --username=admin --password=admin --org=Default_Organization
Error: The service-level command is not supported by the server.

2. Check rhsm.log:
[root@kvm-07-guest01 ~]# tail -f /var/log/rhsm/rhsm.log
2021-07-01 02:41:22,572 [ERROR] subscription-manager:13434:MainThread @connection.py:827 - Response: 404
2021-07-01 02:41:22,573 [ERROR] subscription-manager:13434:MainThread @connection.py:828 - JSON parsing error: Expecting ':' delimiter: line 1 column 11 (char 10)

Actual results:

As step 1, error happened when list available service-level from Sat6.10.

Expected results:
the service-level values should be listed.

Additional info: