We’re fellow users here, so we can’t change the country on your Apple Account for you.
I’d first check whether you still have an Apple Account balance, active subscriptions, pending purchases or refunds, or are part of a Family Sharing group. Any of those can prevent you from changing the country.
Once that’s cleared, go to Settings > your name > Media & Purchases > Country/Region and select Germany. You may also need a valid German payment method and billing address.
Apple explains the requirements here:
Change your Apple Account country or region - Apple Support
If it still won’t let you change the country, what exact message do you see?
One small note: This is the German-language Apple Support Community. Posting your question in German may help you get more responses from other members here.