Thanks for the update @totallyminimad, and for sharing the video of your experience.
Adding a site means you’re site is “Getting ready”, and must be approved by the AdSense team. Sites must be approved before a domain is at the “Ready” stage.
From reviewing your previous screenshot, shared via your Site Health information, it doesn’t look like your parent domain site is at the approved stage yet. Note also that the parent domain is unlikely to be approved is there is no site there for the AdSense team to review, which seems to be the case at present.
What you may wish to do, is add content to the parent domain, so it’s more likely to be approved. Once approved, you will be able to add any subdomain sites to your AdSense account. If using Site Kit you’ll no longer encounter the same error as before, invalid site state null
.
Let me know if that answers your question, or if you have any further questions on this.