Unable to Apply for Child Account

Hello there,

I am sorry but I am stuck in a problem on creating a child account for Google Ad Manager.
I already have an app being approved for monetization, then I went to this page below and tried to click Sign up for a Google Ad Manager account, but after a couple clicks, it does not load anything.

I have read these articles

I appreciate those guides but they do not seem to give the proper approach for this problem.

How do I solve this?

Thank you.
Stay safe and healthy.