You need to wait until the machine is retired before you can submit the write-up. HTB retires a machine every week. Anyone is free to submit a write-up once the machine is retired. The place for submission is the machine’s profile page. Alternatively, if you can’t wait until the machine is retired, you can password-protect your write-up with the root flag like Hackplayers does.
The reason is simple: no spoilers.
Hope this helps.