Msg 443, Level 16, State 15, Invalid use of a side-effecting

Question

hi

i m try write a function in sql server there is a error but not under stand. error is

Msg 443, Level 16, State 15, Procedure get_advice_no, Line 21

Invalid use of a side-effecting operator 'UPDATE' within a function.

Msg 443, Level 16, State 15, Procedure get_advice_no, Line 34

Invalid use of a side-effecting operator 'INSERT' within a function.

my function code is

ALTER FUNCTION [dbo].[get_advice_no](@vcounter_type nvarchar(5) )
RETURNS int
AS
BEGIN
-- Declare the return variable here
DECLARE @vadvice_no int;
-- Add the T-SQL statements to compute the return value here
select @vadvice_no = (SELECT (isnull(CONSTANT_NO,0) + 1) FROM CA_CONSTANT
WHERE CONSTANT_TYPE = @vcounter_type);
if @vadvice_no > 0
begin
---if exits(SELECT CONSTANT_NO FROM CA_CONSTANT WHERE CONSTANT_TYPE = @vcounter_type)
--begin
update ca_constant
set constant_no = @vadvice_no
where CONSTANT_TYPE = @vcounter_type;
-- end
end
else
begin
set @vadvice_no = 1;
IF (@vcounter_type = 'ADV_2')
begin
set @vadvice_no = 75001;
end
INSERT INTO CA_CONSTANT(CONSTANT_TYPE,CONSTANT_NO)
VALUES(@vcounter_type,@vadvice_no);
end
RETURN(@vadvice_no);
-- Return the result of the function
END

pls tell me what is wrong