Hi @MeganDear ...
Thank you for the reply. Unfortunately, your process does not sound familiar to me...at least not a process that I'm familiar with as it relates to the Canvas UI (User Interface). There is no "out-of-the-box" Agenda tab. Also, if you are working within the "Assignments" course navigation menu, there isn't a way to click on "insert" >> "document" >> "upload document" (at least not from the "Assignments" index page). So, I am still a bit confused as to where you are working within your Canvas course. Maybe a screenshot or two might be helpful? Looking forward to hearing back from you soon.
EDIT: Maybe you are trying to edit an assignment using the RCE (Rich Content Editor) and using the "Insert" >> "Document" >> "Upload Document" option in that menu? If so, have you tried uploading your document first to your "Files" area of your course? Then, in the RCE, you could choose "Course Documents" instead of "Upload Document" and find/select the file you had uploaded to your "Files" area of the course. Either of these Guides should be helpful:
Keep us posted here in the Community...thanks!