--so this is bad:
EXEC msdb.dbo.sp_send_dbmail 
 @subject = 'Please send Feedback for Order#' + Orders.OrderNum 

--this is the correct way:
declare @var varchar(128)
SET @var ='Please send Feedback for Order#' + Orders.OrderNum 
EXEC msdb.dbo.sp_send_dbmail 
 @subject =@var 

DECLARE
  @isql VARCHAR(2000),
  @email VARCHAR(64),
  @order VARCHAR(64),
  @emailSubject varchar(128),
  @emailBody varchar(128)
  DECLARE c1 CURSOR FOR 
    SELECT 
      CustomerEmailAddress,
      OrderNum
    FROM Orders 
    WHERE EmailSent = 0

  OPEN c1
  FETCH NEXT FROM c1 INTO @email,@order
  WHILE @@FETCH_STATUS <> -1
    BEGIN
    SELECT 
      @emailSubject = 'Please send Feedback for Order#' + @order,
      @emailBody = @emailSubject --you'd build a bigger, more descriptive string for the body.

      --you cannot concatenate strings  as the parameters for a query, they must be eitehr a static string or an already built variable.
      EXEC msdb.dbo.sp_send_dbmail 
      @profile_Name ='Administrator',
       @recipients= @email ,
       @subject = @emailSubject,
       @body = @emailBody

    
    FETCH NEXT FROM c1 INTO @email,@order
    END
  CLOSE c1
  DEALLOCATE c1

SELECT @EmailBody = EmailTemplate 
FROM Campaigns WHERE  templateID = 42
--assuming there are customized fields for find and replace?
SET @EmailBody = REPLACE(@EmailBody, '[firstnamePlaceholder', @FirstNameFromCursor;