iCloud verification code, even though no other devices

After erasing the hard drive and reinstalling Mac OS , if you try to set up iCloud Keychain a trusted device ( its could be your iPhone or an android phone ) is required to be configured in iCloud Keychain , so that you get a six digit iCloud Keychain code on the iPhone or incase you are using an android phone and that has to be filled up , see this article https://support.apple.com/en-us/HT204085

Also your Apple ID and password , the web mail address will be the same as you had already created in the past on the same Mac and and that are written with you , only after reinstallation the iCloud Keychain is to be set up .

Note : its not necessary that in the past you used a different mobile number ( trusted device )to set up iCloud Keychain , after reinstallation of Mac you can configure different mobile number .

See this article https://support.apple.com/en-in/HT202861

On your Mac using OS X Mavericks v10.9 or later

1. Choose Apple () menu > System Preferences.
2. Click iCloud, then select Keychain.
3. Enter your Apple ID password.
4. Click Advanced, then choose one of these two options:

• • Get a Random Security Code. 
  • Don't Create Security Code. If you don't create an iCloud Security Code, then your iCloud Keychain is stored locally on your Mac instead of Apple's servers, and will update only across your approved devices.

1. Follow the onscreen instructions to complete iCloud Keychain setup.

There are two options .

( a ) set up iCloud Keychain on Mac a verification code will appear and you have to purchase a mobile phone to configure .

( b ) if you are not having a mobile phone don't create a code and no iCloud verification code will be saved on iCloud server .

In your case you have to reset the iCloud Keychain https://support.apple.com/en-in/HT202755

This will delete all the passwords saved in iCloud Keychain , make sure your passwords are written with you before performing this step .

Also consult senior apple care advisors see this link https://support.apple.com/en-in/HT201232

Read this https://support.apple.com/en-us/HT204085

https://support.apple.com/en-in/guide/mac-help/use-keychains-to-store-passwords-mchlf375f392/10.14/mac/10.14

Note : you can click on cancel button , to set up iCloud Keychain a mobile phone is must and without configuring it iCloud Keychain ( the box of keychain will not get checked ) .