It seems that you need to improve your SQL skill, Please read google SQL and group by clauses.
Apart from that you can have two solutions.
1. Remove your Group by Clause at all Like this
select distinct si.Date, si.SalaryId,er.EmpCode, er.FName from tbl_EmpRegistration er,tbl_SalaryInformation si where er.EmpCode=si.EmpCode and si.PartnerName='Pio' and er.PartnerName='Pio';
2. USe Group by with modifications in select clause
SELECT T.Date, T.MaxSalaryID SalaryId, er.EmpCode, er.FName
FROM
(
SELECT si.Date, si.EmpCode, MAX(si.SalaryId) MAXSalaryID
FROM tbl_SalaryInformation si
Group by si.Date, si.EmpCode
) T INNER JOIN tbl_EmpRegistration er on T.EmpCode = er.EmpCode