It's most likely due to a missing CAPTCHA which would ask you to place a checkmark where it says "I am not a robot." At least some users have to place the checkmark in order to continue.
I know you've already tried cleaning the browser. Try these additional steps and then look somewhere on the end efiling screens for a CAPTCHA with "I am not a robot."
Log out and empty the browser Internet cache. Close the browser and reopen it. Then try your browser in private browsing mode (aka "incognito mode" in Chrome). ; i.e., open a private window, then log into TurboTax again and try to efile. If that doesn't help, then try a different browser to see if it recurs.
Please report back with your results
