If you finished the rest of your onboarding process but did not receive info about the background check, don't wait before contacting support.
I wish there was official communication about this, because it's a widespread issue that they are aware is happening, and they are able to fix it quickly and trivially on an individual basis.
If you did not get an email from Certn to begin your background check within 24 hours after completing the rest of your onboarding, contact Alignerr support via the widget on the bottom right of the Alignerr website - the page where you see your list of exam pass/fail results.
You may need to interact with the support chat bot first. The bot does not immediately give you a text box to type in a custom question. Instead, you need to click one of the 4 buttons presented to you to start a conversation. I chose [Billing], the bot spit out a copypaste answer about billing, and then gave me a text box for custom questions. This is where you need to steer the conversation in a way that the bot ultimately can't help you, and it puts you in the queue to be picked up by a person.
The bot itself cannot email you the Certn background check; a real person has to take over your conversation. You cannot tell that you were put in the queue to be helped by a person. You will only know when they respond to you, some days later.
My advice is to keep asking the bot how to contact Alignerr support, without actually explaining your problem. Ask for the support email (it won't give one to you). Do not explain your problem to the bot, because it will give you an unhelpful answer and mark your conversation as solved.
Once a human understands that you didn't get the Certn email, they send it instantly, you complete it in a few minutes, and the results process in a matter of hours. You get your Labelbox and Slack invitation the same day.
Do not wait. If you did not receive an email from Certn after doing the rest of your onboarding, waiting is not going to do anything: the email is not coming, and you must ask support to send it to you.