Using self-assessment gov't gateway to register for child benefit

I'm trying to register my newborn for child benefit using my HMRC self-assessment government gateway, and got this message: "An organisation Government Gateway account cannot be used to make a Child Benefit claim. Use a personal Government Gateway user ID and password." I do own a limited company and have a business tax account that's separate from my self-assessment account, but I am absolutely certain I'd used my self-assessment account instead of the company's. In fact, I had tried both and got the same message both times. In case it's important - I plan to elect to not receive child benefit for tax purposes, but would still like to register my newborn using my personal tax account to earn NI credit. How do I register for child benefit so I can earn NI credit when the system wouldn't let me use my self-assessment government gateway to do so?

Hi C L-W,
You will need to enrol for personal tax account for individuals to register a new born child.
Please see guidance here: 
Personal tax account: sign in or set up
Thank you.