SQL Server: how to do if statement within the insert statement

You need to do your check first, before you then either run your INSERT, or skip it. You cannot have IF statements in the middle of an INSERT:

IF NOT EXISTS(SELECT * FROM dbo.SDOPTTABLE WHERE OPTTABLENO = '324' AND OPTTABLEVAL = '00883578')
    INSERT INTO HCSERVREC (COMP_CD, JOBTL_CD, emp_no,from_dt,service_type)
    VALUES ('1', .......)

And the INSERT statement can take two forms - either you have a fixed list of values as literals ('1') or SQL variables - then you can use the INSERT INTO Table(columns) VALUES(values) approach.

Or you want to use a SELECT statement from another table to insert the values, then you need to use the INSERT INTO Table(columns) SELECT column FROM OtherTable WHERE condition style. Again: you can pick one or the other - but you cannot mix them (e.g. you cannot have a sub-SELECT in the middle of a VALUES(., ., ....) enumeration

I would suggest that you do this as a single statement, using insert . . . select:

INSERT INTO HCSERVREC(COMP_CD, JOBTL_CD, emp_no,from_dt,service_type)
    SELECT '1', '324', '00883578', 'STOCKROOM ATTENDANT', getdate()
    WHERE NOT EXISTS (SELECT 1
                      FROM SDOPTTABLE
                      WHERE OPTTABLENO = '324' AND OPTTABLEVAL = '00883578'
                     );

That's not possible. While inserting you need to get only one value for one column.

Either as suggest first check not exist select query and then insert.

yes, you can use case or subquery to get a single value to replace if statement above.

updated

If you wish this type of situation, then you can create dynamic query for this.

--declare table1 table ( id int, value varchar(10) )
--drop table Table1
begin tran
create table table1 ( id int, value varchar(10), value1 varchar(10), value2 varchar(10))
insert into table1 values( 1,'001','A','A1')
insert into table1 values(2, '002','B','B1')
insert into table1 values( 3,'003','C','C1')

-----insert example
--always use nvarchar for dynamic query, other wise gives error
declare @sql nvarchar(max), @columnlist nvarchar(max) = 'id',  @columnlistvalue nvarchar(max) ='4'
declare @checkvalue varchar = '1',@checkvalue2 varchar = '2',@checkvalue3 varchar, 
@columnvalueFirst varchar(10) = 'this entry done by dynamic query on condition basis'

if(@checkvalue = 1)
  begin
    set @columnlist = @columnlist + ',value'
    set @columnlistvalue = @columnlistvalue + ',''004''' --or you can add @checkvalue
  end
if(@checkvalue2 = 2)
 begin
    --this is example, you can add another condition. Right now if else condition gives same output
    if( len(@columnlist) > 0)  
    begin
        set @columnlist = @columnlist + ',value1'
        set @columnlistvalue = @columnlistvalue + ',''D''' --or you can add @checkvalue2
    end
    else
    begin
        set @columnlist = @columnlist + ',value'
        set @columnlistvalue = @columnlistvalue + ',''D'''
    end
 end    
if(@checkvalue3 = 3)
 begin
    --this is example, you can add another condition. Right now if else condition gives same output
    if( len(@columnlist) > 0)  
    begin
        set @columnlist = @columnlist + ',value2'
        set @columnlistvalue = @columnlistvalue + ',''D1''' --or you can add @checkvalue3
    end
    else
    begin
        set @columnlist = @columnlist + ',value2'
        set @columnlistvalue = @columnlistvalue + ',''D1'''
    end
 end

select * from table1
set @sql = 'insert into table1 ('+ @columnlist +') values ( ' + @columnlistvalue + ')'


    select @sql  --you can check that what is wrong by 

exec sp_executesql @sql
select * from table1    

rollback