i had similar problems today 3sep2020 3pm and 11-1am uk time, and previously
all stop orders were rejected, market orders were rejected, with the same message:
"the market you have requested is no longer available online"
heppened on the website and on android mobile app
happened with zoom,twitter,alphabet,amazon,apple
the system only accepted limit orders to sell, but even those failed to trigger (may be no buyers !?)