A family of Microsoft word processing software products for creating web, email, and print documents.
First verify that German is indeed installed as a proofing language. You can tell if you look in the Language dialog box (Review tab > Language group > Language > Set Proofing Language). Each installed language has a check mark next to it.
If German is not installed, it needs to be installed. This can normally be done via https://support.microsoft.com/en-us/office/languages/language-accessory-pack-for-microsoft-365 but since you are in an organization, your local admin may have to do it for you.
Once you know that you have support for German, you also need to select all the text and (re-)apply the language to the selection. This can be done in the Language dialog box (select the text before you revisit the dialog). As has been suggested, "Do not check spelling or grammar" must be cleared, as well as "Detect language automatically."