Invio, ricezione e organizzazione della posta elettronica in Outlook.com
Hello
You said it is an old email address.
When was the last time you used this email address? If you haven't used this account during the last two years, it is unfortunately closed permanently and cannot be recovered. Microsoft's account activity policy is explained here: https://support.microsoft.com/topic/microsoft-account-activity-policy-7c0a9fa7-0982-b7c6-fd72-df852b63699f
Microsoft preserves every username that's been used, even if the account is permanently closed, the username cannot be reused, this is why it says the account already existed when you tried to recreate.
If you can tell me when the last time you used the email address, I can determine if there's any way at all you might be able to recover it.