Hi @Matthew Manier
Could you please tell me if the user is a guest user of the organization?
If the user is a guest user, the file sharing issues are occurring because the settings were not made correctly in the SharePoint admin center.
To fix this issue, configure the sharing settings in Teams and SharePoint as follows:
1.Enable group owners to add people outside the organization as guests.
When you create a team, a Microsoft 365 group is created to manage team membership. To let the group owners add people outside the organization to the group as guests, select the Let group owners add people outside the organization to groups check box from Microsoft 365 Groups guest settings.
2.Enable guest sharing from SharePoint admin center:
Sign in to the SharePoint admin center as an administrator.
In the left navigation pane, select Policies > Sharing.
Specify the sharing level for SharePoint and OneDrive to one of the following options:
Existing guests
Anyone
New and existing guests
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.