To prevent users from deleting messages posted in private chats and channels, you need to manage the messaging policies in Teams. Here are the steps to follow:

Sign in to the Teams admin center with your admin credentials.

Go toTeams apps>Messaging policies.

Select the policy that you want to modify, such as the global (Org-wide default) policy or a custom policy.

Turn off the following settings:Owners can delete sent messages,Users can delete sent messages, andUsers can edit sent messages.

ClickSaveto apply the changes.