I cannot register email address using my own domain

I cannot register email address using my own domain

I had previously registered admin@mydomain.com a few days ago on Zoho Mail, but then changed my mind and decided to use Proton. I then impulsively deleted my entire Zoho account. But later, I discovered Zoho forms and created a Zoho account again. I thought it would be better to have my email in Zoho as well since I’m using Zoho forms, so I once again set up an email. But this time, Zoho won’t let me register the same email address admin@mydomain.com

I have checked my DNS settings (I use Cloudflare), and I see no records for that email. Please help. I have contacted Zoho Support about this, but there has been no response. Not even an email to confirm that they've received my support request, so I wonder if they even got my email at all