How can I approve a new iPhone without an old iOS device passcode?

Can I guess that the two factor authentication is asking for the passcode to one of the devices on your Apple account that you no longer have but forgot to remove from your account?

Best to contact Apple Support directly. Depending on your region, you may be able to reach them here: Get Support

After you have this sorted out make sure you have another method of receiving auth codes like another trusted friend’s mobile 

https://support.apple.com/en-us/HT204915

 you need to log into icloud.com and make sure the other devices are removed from your account.

https://support.apple.com/en-us/HT204074

There is also a great answer here

https://discussions.apple.com/thread/252534968