Hi @Sykesy Sykes ,
Are there any error messages around the red box?
Do you have any attachments in your uploaded files?
What is your format of your uploaded file?
Firstly, we recommend you try to login in Teams web client and upload the file again to see if this issue can be reproduced. If this issue still persists, we suspect that issue may be caused by SharePoint site permission. You may contact team owner, let him login SharePoint and check that site permission settings. For more reference:
https://learn.microsoft.com/en-us/sharepoint/customize-sharepoint-site-permissions
In addition, you can try to refer the following article to upload the assignments to see if it works for you.
https://wymondhamcollege.s3.amazonaws.com/uploads/document/How-to-Upload-Work-on-Microsoft-Teams-1.pdf?t=1588187106
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.
If the response is helpful, please click "Accept Answer" and upvote it.
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.