Files you share in a channel are stored in your team's SharePoint folder, which is found in the Files tab at the top of each channel.
Files you share in a private or group chat are stored in your OneDrive for Business folder, which is found in the Files tab at the top of a chat. These files are only shared with the people in that conversation.
You have some license prerequisites
https://docs.microsoft.com/en-us/microsoftteams/sharepoint-onedrive-interactIf you can't figure it out you should open up a support ticket with Microsoft. If you're not an admin yourself reach out to that person for assistance.