The credentials you provided for the SQL Server Agent service are invalid. To continue, provide a valid account and password for the SQL Server Agent service.

In SQL Server silent installation or installation with configuration file this error is very common when we forget to give password for SQL service account.
In my installation i was passing below command in command prompt >> setup.exe /CofigurationFile=D:\saurabh\ConfigurationFile.ini /QUIET="TRUE"  /SAPWD="saurabh"

Which resulted in error:
" The credentials you provided for the SQL Server Agent service are invalid. To continue, provide a valid account and password for the SQL Server Agent service."

The easiest way to avoid this error is give password manually by adding below command
/SQLSVCPASSWORD="************" /AGTSVCPASSWORD="************" /ASSVCPASSWORD="************" /ISSVCPASSWORD="************" /RSSVCPASSWORD="************"


So syntax will become:
>> setup.exe /CofigurationFile=D:\saurabh\ConfigurationFile.ini /QUIET="TRUE"  /SAPWD="saurabh" /SQLSVCPASSWORD="sql engine service password" /AGTSVCPASSWORD="agent service password" /ASSVCPASSWORD="Ananlysis service password" /ISSVCPASSWORD="*Integration service password" /RSSVCPASSWORD="Reporting service password"