We have been made aware of some issues opening Word documents on the desktop version of Microsoft Teams. Documents are opening in preview rather than edit mode.
If this happens to you, please use the web version of Teams to open your Word documents until Microsoft can resolve this issue.
Others are reporting this issue as well. See https://answers.microsoft.com/en-us/msoffice/forum/all/cannot-edit-word-documents-in-teams/7befc8f8-b262-4331-a2f6-da6ea019edf1