First, make sure that your document is in PDF or JPEG format and that it is no more than 10MB.
You can upload files from your computer or from your personal Google Drive space.
From your computer: Click on “Select files from computer”, choose the desired document and click on “Open” or “Import”
From your personal Google Drive space: Click on “My disk” choose the desired document and click on “Select”
If your storage space is full in your personal Google Drive space, this may be the cause of the problem.