When installing SharePoint 2016 from PowerShell, make sure you use the machineName\userName when prompted.

 

Example:

New-SPConfigurationDatabase –DatabaseName SharePoint_Config –DatabaseServer machineName\sqlInstance –AdministrationContentDatabaseName SharePoint_Content –Passphrase (ConvertTo-SecureString BigTaco@12345 –AsPlaintext –Force) –FarmCredentials (Get-Credential) -localserverrole SingleServerFarm

A login prompt will appear.

for the username: enter your machine name \ username (WIN-SP16\spUser).

Error I received when I tried to use spUser without the machine name.

new-spconfigurationdatabase the user does not exist