I ran into an issue today while installing AAD Sync using command line parameters to designate an external SQL Server. The error message was:
Error 25001.The Microsoft Azure AD Sync setup wizard cannot validate the information for service account, password, or domain or local computer. Verify the entered information is correct, and then try again.
Basically, the problem was that I had some special characters in the password which required double quotes. The command line that worked perfectly is:
directorysynctool.exe /sqlserver <SQL Server hostname> /serviceaccountdomain <NETBIOS_domain_name> /serviceaccountname <service_account> /serviceaccountpassword "<password>"
directorysynctool.exe /sqlserver sql.contoso.com /serviceaccountdomain CONTOSO /serviceaccountname AAD-Sync /serviceaccountpassword "Password1"
Anyway, hopefully this will help someone who runs into this situation.