The quality of the gov.uk website is worrying me. For sure I am not the only person annoyed by the problem I am describing bellow.
I have been trying in the last hours to apply for my son child benefit under my name, while in the last years was under his mother name.
The government application has many moment when it doesn't do anything when clicking button while completing the form and because I have web developer knowledge I opened the developer tools in Chrome and looked under the network tab. A normal person will just look at it and do not understand what is happening. Hopefully, my post will make you understand better the problem you have and how to track it.
The request from the bellow image is done when I click the submit button and is returning an error but the error is not printed on the page because it was too much for the developer who worked on this functionality, to do.
By looking on the response tab I found also a logic for why is not working. It seems that the application is locked for the moment.
Because my child mother cancelled the child benefit a couple of hours before I started my application it is possible that the gov servers are not fully updated yet so they still see the child benefit under his mother name. See "Resource is locked by another user" message
As an advice, and this is what I will do too, is to wait for a few more hours and maybe try tomorrow to see if I can finish the application.
Don't forget to Save and continue later your form to do not lose the details you already inserted.