Because I don't know what You want to do. How to relate SiMain to SisUp?
1. All records from SiMain & only one record from SisUp that match record from SiMain
or
2. All Records from SiMain & all records from SisUp that match record from SiMain

for 1. old syntax must work well

    SELECT SiMain.*, SisUb.ICode, SisUb.qntymaj, , SisUb.qntymin, , SisUb.RateMaj, SisUb.RateMin,;
           PCode.PName, Icode.full_desc;
           FROM SiMain;
           LEFT JOIN SisUp ON SiMain.SiNo  == SisUp.SiNo; && May be You need RIGHT JOIN here?
           LEFT JOIN PCode ON SiMain.PCode == PCode.PCode;
           LEFT JOIN Icode ON SisUp.Icode  == Icode.Icode;
           WHERE BETWEEN(Simain.Invdate, mdt1, mdt2)        .AND.;
                 UPPER(Simain.Salesman) == UPPER(msalesman) .AND.;
                 Simain.PCode == mpcode;
    INTO CURSOR cTest

for 2.

    SELECT SiMain.*, SisUb.ICode, SisUb.qntymaj, , SisUb.qntymin, , SisUb.RateMaj, SisUb.RateMin,;
           PCode.PName, Icode.full_desc;
           FROM SiMain;
           RIGHT JOIN SisUp ON SiMain.SiNo == SisUp.SiNo; 
           LEFT JOIN PCode ON SiMain.PCode == PCode.PCode;
           LEFT JOIN Icode ON SisUp.Icode  == Icode.Icode;
           WHERE BETWEEN(Simain.Invdate, mdt1, mdt2)        .AND.;
                 UPPER(Simain.Salesman) == UPPER(msalesman) .AND.;
                 Simain.PCode == mpcode;
    INTO CURSOR cTest

BTW Is the Report running?