Catalina Verification Failed

Are you referring to a file verification error during or immediately after you downloaded the installer? If so, that means the file is corrupted. Be sure you are using a fast Wi-Fi or an Ethernet connection to the Internet. The latter is preferred. If you are using a slow Wi-Fi, then glitches during transfer are more common and can cause file corruption. Exactly, which iMac model are you using?

If the above doesn't help, then you will need to be more specific about what you or the computer was doing when the error occurred. Please read Writing an effective Apple Support Communities question. We are here to help. We are all users. We are not mind-readers nor clairvoyants. We only know what you write down and tell us. Be specific and thorough. Include your hardware information and installed version of macOS.

2FA means Two-Factor Authentication. It is a security measure. Two-factor authentication for Apple ID - Apple Support.

To what verification are you referring? Do you mean 2FA? Normally, an upgrade doesn't require verification. It requires your Apple ID and password. Learn to compose a better topic question: Please read Writing an effective Apple Support Communities question. We are here to help. We are all users. We are not mind-readers nor clairvoyants. We only know what you write down and tell us. Be specific and thorough. Include your hardware information and installed version of macOS.