Given that it already asks for your code you must have previously set-up the Google Authenticator for your Mx account. Maybe for an older phone? If you do not have your Mx account in your current phone's Google Authenticator app you will need to request Mx support at https://support.mendix.com to reset the two-factor authentication for your account.
This is a simple support request so usually done within a few minutes to hours.
After that you may go through the setup again and you will be able to continue changing the technical contact.
You can follow the steps in this document, https://docs.mendix.com/developerportal/deploy/two-factor-authentication#:~:text=Open%20the%20Google%20Authenticator%20app,%2Ddigit%20time%2Dbased%20code.