The most common reason why you cannot upload a file to a Shared Folder or a Group Folder in the Drive is not having the required permissions. When uploading files to Shared Folders and Group folders you will need to have Moderator, Admin, or Owner privileges; Members are not able to upload files.
Other less common reasons may include:
• The size of the file exceeds 50MB
• The file type is not supported
• You have exceeded your storage limit of 3GB
• The internet connection is weak or lost