Recurrence is not available for the multiple tasks schedule. It is only available for singular scheduled tasks. You have two options for accomplishing your goal:
1. Manually schedule in the multiple tasks schedule all of the desired run dates/times.
2. Add a new/separate row for each task that you want to be recurring. Create a singular scheduled task with recurrence for each row.
If you have a grid synchronization task that is being used to remove other rows, you can prevent desired rows from being removed by either using ‘Synchronize grid with directory (add hosts only)’ so that your synch task never removes any hosts, or if you want to remove hosts but just don’t want to remove particular hosts then use the ‘Exclude hosts from synchronizations’ option that is available in the lower right corner of the Grid Synchronization Settings window.