Hmm, that's odd. Can you share the custom URL that you want for your channel? Normally, once one is generated you can't change it back, but you can remove it and claim a new one up to 3 times per year. More info here: https://yt.be/help/4moa
Hi there. your desired URL AllAquaBeats has been reserved for your YouTube Channel. The custom URL now only needs to be accepted. To do this, go to http://youtube.com/account_advanced …. You should see a notification about the available custom URL. Follow the link and accept the URL.