First, be sure the 6-digit code is not ending up in your email's spam/junk folder or another email address you have.
See this FAQ on some tips, which also includes info on how you can change the email address in your Account Settings, if you wish to use another email address.
Remember to keep the tab/window open that's asking you for the code while you go to your email to get the code. If you close that original tab/window, it resets everything, and the code would be seen as invalid. Use a second tab or second window or a different browser to check your email. Or use your phone to check your email for the code while leaving the computer window open.
FAQ: What if I'm having trouble with my filing verification code?
https://ttlc.intuit.com/community/security/help/what-if-i-m-having-trouble-with-my-filing-verificati...
