Try this....
SELECT Distinct ti.TicketID, ti.UserID, ti.AttachFile, ti.HasFile, ti.Title, ti.Question, ti.Flag, ti.InsertDate, tb.BranchName, ta.AnswerID, ta.Answer, tu.UserName, tu.UserOwner, tu.Corporation
FROM dbo.TBLTicketBranchs tb
INNER JOIN dbo.TBLTickets ti ON tb.BranchID = ti.BranchID
INNER JOIN dbo.TBLUsers tu ON ti.UserID = tu.UserID
LEFT OUTER JOIN dbo.TBLTicketAnswers ta ON ti.TicketID = ta.TicketID
WHERE tb.ResellerID = @ResellerID
AND ( ta.Answer LIKE N'%' + @Keyword + '%'
OR @Keyword IS NULL
OR ti.Title LIKE N'%' + @Keyword + '%'
OR @Keyword IS NULL
OR ti.Question LIKE N'%' + @Keyword + '%'
OR @Keyword IS NULL
OR tu.UserName LIKE N'%' + @Keyword + '%'
OR @Keyword IS NULL
)
Try using Alias Names While Writing Joins...