Thank you for your post!
As of right now, Microsoft Graph for US Government L4 (https://graph.microsoft.us) and Microsoft Graph for US Government L5 (DOD - https://dod-graph.microsoft.us) are not supported within Graph Explorer.
For an app in US Government:
Access to US Government data using the worldwide endpoint will be disabled in the near future. For more info - Microsoft Graph and Graph Explorer service root endpoints
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.