That is normal and not a problem. Members of the U.S. forces are exempt from the German registration requirement, so most never had one.
Providers ask for an Abmeldebescheinigung because that is what a German civilian would have. When a clerk asks for it, they are following a script, not the law.
Your orders serve the same purpose: they prove you are leaving and when.
Submit your orders instead.
Say plainly that you are a member of the U.S. forces under the NATO Status of Forces Agreement and exempt from registration.
If a provider insists, your installation's legal office can usually issue a confirmation letter.
That is normal and not a problem. Members of the U.S. forces are exempt from the German registration requirement, so most never had one.