A few of our students have had difficulty submitting an assignment in Canvas. With the help of Canvas support, we have discovered that the LastPass extension is at fault. When a student submits an assignment with the LastPass extension installed, the file is uploaded (ie: it appears under "Files you have already uploaded"), but the "Submit Assignment" button stays grayed out and the submission never appears in SpeedGrader.
Available workarounds are disabling the extension or loading Canvas in an Incognito window.
I haven't had much luck with the LastPass settings, or with the information available in the browser Developer Tools, but it could be that my Jedi skills aren't strong enough.
Hopefully the community might have some insight