(I'm using Canvas in laptop) I don't know how to report a bug but I think I found a bug in file submission. I accidentally click the submit assignment button without attaching the file first, so canvas notified me that it can't pass an empty submission. However, as I tried to click "Upload File", the button does not work, it just become unresponsive no matter how many tries I do it. So I went back to dashboard, and click the course again and choose my assignment, this time I clicked the button "Upload File" and now the button works. 

To verify if the bug is really consistent I tried it again in different assignment, I click the submit assignment without attaching anything, after the notification I tried to click the button "upload file" and it doesn't work again.

I hope this problem will be solved, good luck to the developer team of Canvas. Have a great day!