Spaces, my friend, spaces. As in you need one between
ClientTbl.ClientID
and
WHERE(RentalTbl
da = new SqlDataAdapter("SELECT RentalTbl.NumberPlate, RentalTbl.PricePD, RentalTbl.FromDate,"
+ "RentalTbl.ToDate, ClientTbl.CompanyName, ClientTbl.ContactName, ClientTbl.Mobile,"
+ "ClientTbl.Email FROM RentalTbl INNER JOIN ClientTbl ON RentalTbl.ClientID = ClientTbl.ClientID "
+ "WHERE(RentalTbl.NumberPlate = @Param1)", conn);