I'm not able to find a specific answer via Google search but this problem seems to be a common one. Here's what I got so far – https://support.google.com/business/answer/4578106?hl=en. And here's how you can 'report' – https://support.google.com/maps/answer/3094088?rd=1#report. Hope this helps.
Ask Google to resend you a letter with confirmation code, this should help (After you do what Content Maestro adviced)