You can enter the MT4 trading platform using the contest login and password only after the round has begun.
First of all, сheck you’re trying to enter the right trading platform.
The Champion Demo Contest is only available on MT4.
Then check if the contest round is Active.
You can find this information on the main page of your Personal Area.
You can see the statuses Not yet started, Active or Finished in the same rows as the MT4 contest accounts.
You can log in to the MT4 trading platform and start trading only if the contest account is marked as Active.
For more information please contact OctaFX support team from the official website.