The VSA login page looks for a folder on the Kaseya server called C:\Kaseya\WebPages\AuthAnvil\inc. During installation of the Kaseya logon agent / Kaseya addin, if the AuthAnvil passcode prompt was set to be turned off, it will rename the "inc" folder to "incBackup". This hides the folder from the login check, disabling the AuthAnvil passcode prompt and not requiring 2FA protection for any users.
You can enable the passcode field at any time by logging onto the Kaseya server and renaming the "incBackup" folder located in C:\Kaseya\WebPages\AuthAnvil to "inc". If you want to manually disable it and turn off the 2FA protection, manually rename the "inc" folder to something different like "incBackup".