Use sandbox account as regular account(Apple). How to deal with itunes account creation not allowed app store error?

Question

I created Apple Id account and initially used for testing purchases as a sandbox account. For now, I want this account to use as a regular account. So, I deleted it from the sandbox users list, logged into the device, but I cannot install any of apps. The annoying dialog appears: iTunes account creation not allowed

How can I deal with that dialog?

How can I transform the sandbox account for regular using?

Why do I get this error?