Sonia,
Long shot here but after clicking the button did you scroll down the page? The area to submit assignments and towards the bottom of the page. On my computer I almost always have to scroll to even see it and at times have thought it wasn't there because of it.
Another potential issue could be related to installed browser extensions. I've never heard of one causing that exact issue but I have seen an extension prevent a Submit button from even being displayed. If you want to test that scenario trying opening an Incognito window in Chrome and test it out. When in incognito mode any installed extensions should be disabled. If it works that way there is a good chance that a browser extension is causing the issue.
Rick
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