Conversion failed when converting the varchar value ' and f.FUNCTION_ID= CAST(' to data type int

I am trying to make the procedure where I can get the data from MS SQL. Here I am getting the error

<b>Msg 245, Level 16, State 1, Procedure GET_FUNCTIONS, Line 15 [Batch Start Line 33]
Conversion failed when converting the varchar value ' and f.FUNCTION_ID in '' to data type int.</b>

    CREATE procedure [dbo].[GET_FUNCTIONS]
    @Function_Id int,
    @EntityId int,
    @OrderBy varchar(10)
    as
    begin
    declare @Sql nvarchar(max)
    if(@Function_Id=0 AND @EntityId=0)
    set @Sql = 'select                                                        
    f.Function_Code,f.FUNCTION_ID,f.EntityId,be.ENTITY_SHORT_NAME,f.FUNCTION_NAME,
    f.FUNCTION_DESC,f.CREATED_DATE,f.MODIFIED_DATE from FUNCTIONS f 
    inner join BusinessEntity be on f.EntityId=be.ID
    WHERE f.ACTIVE=1 '

    if(@Function_Id!=0)
    set @Sql += ' and f.FUNCTION_ID in ''' + @Function_Id + ''''
    if(@EntityId!=0)
    set @Sql += ' and f.EntityId = cast('+convert(varchar(12) ,@EntityId)+') '

    set @Sql += ' order by FUNCTION_ID '+ @OrderBy
    print @Sql
    end
    GO

Thanks everyone for your answers. I have got the answer by my own.

I have removed if(@Function_Id=0 AND @EntityId=0) and changed the data type of @Function_Id and @EntityId to varchar(100), so solved all errors.

I'm, making a few guesses here, which I add some comments about, however, this should be what you are after. I firstly ensure that I parametrise the values, but also validate and quote the value of @OrderBy:

CREATE PROCEDURE [dbo].[GET_FUNCTIONS] @Function_Id int =NULL, --Instead of passing 0as "select all" use NULL
@EntityId int = NULL, --Instead of passing 0as "select all" use NULL
@OrderBy nvarchar(4) --guessing this is mean to be ASC or DESC?
AS
BEGIN
IF @OrderBy NOT IN (N'ASC','DESC')
--Error is not ASC or DESC
THROW 51473, N'@OrderBy can only have a value of ''ASC'' or ''DESC''.',16; --51473 was a random error number; you can use one that is appropriate for your environment.
DECLARE @SQL nvarchar(MAX),
@CRLF nchar(2) = NCHAR(13) + NCHAR(10);
SET @SQL = N'SELECT f.Function_Code,' + + @CRLF +
N'       f.FUNCTION_ID,' + + @CRLF +
N'       f.EntityId,' + + @CRLF +
N'       be.ENTITY_SHORT_NAME,' + + @CRLF +
N'       f.FUNCTION_NAME,' + + @CRLF +
N'       f.FUNCTION_DESC,' + + @CRLF +
N'       f.CREATED_DATE,' + + @CRLF +
N'       f.MODIFIED_DATE' + + @CRLF +
N'FROM FUNCTIONS f' + + @CRLF +
N'     INNER JOIN BusinessEntity be ON f.EntityId = be.ID' + + @CRLF +
N'WHERE f.ACTIVE = 1';
IF (@Function_Id IS NOT NULL)
SET @SQL = @SQL + @CRLF + N'  AND f.FUNCTION_ID = @Function_Id'
IF (@EntityId IS NOT NULL)
SET @SQL = @SQL + @CRLF + N'  AND f.EntityId = @EntityId'
SET @SQL = @SQL + @CRLF +
N'ORDER BY F.FUNCTION_ID ' + QUOTENAME(@OrderBy) + N';'
--PRINT @SQL; --Uncomment for debugging.
EXEC sp_executesql @SQL, N'@Function_Id int, @EntityId int', @Function_Id, @EntityId;
END;
GO