*By default Administrator account is disabled in Windows Server 2016.
You might have used some name while promoting "Server" (Stand-Alone) to "Primary" (Domain Controller). You have to use that name and password associated with it.
You can use "Offline Registry" to know the name and change the password, net use command will help you do that. Watch this video to fix this issue.
In addition to above, you can use Active Directory user to access the server, you will still have user rights not Admin rights. However, try to login and open "Active Directory Users and Computer" and select "Users" container you will find "Default Administrator" account disabled, there you have to find the name you chose while promoting "Server" (Stand-Alone) to "Primary" (Domain Controller) use that name to access DC, if you remember the password.