It might help to know which course you are taking. It sounds like you’ve already found some information about possible causes that is equivalent to this FAQ Topic.
Has this worked for you on previous assignments or is this the very first time you’ve tried an assignment? If the latter, then it may be a permission problem: e.g. the operation is being blocked by A/V software on your computer or a firewall or proxy server in your local area network. The Submit button is the last thing to render when you open a notebook page in the browser. You might want to try a different computer, different browser or try from a different network. E.g. if you are at work, try from home or from a public WiFi and see if that makes a difference.