For some obscure reason that only Samsung's Android developers would know, it has never have the ability to define bi-weekly or fortnightly repeat event or let along repeating task.
My latest NoteEdge (SM-N9150) running Android 6.0.1 still does not have it. In the process of finding a third party reminder app to supplement the deficiency in S Planner, I have discovered a very simple way to do this.
To allow you to define custom repeat, you install the "To-Do Calendar Planner" which install the isoTimer app into your handset.
When you start the isoTimer for the first time grant it permission to access your Calendar. You can deny it permission to your Contact just as I do.
Then you use the isoTimer's interface, albeit a bit unusual, to create an event or task and to set bi-weekly repeat use the "Repeat every X Days" option.
What this program does is to inject those repeats into the S Planner's Calendar. I am using a localised calendar as the default and that is where the isoTimer injects the repeat event/task into.
So it seems Samsung has stubbornly refused to implement an user interface to support bi-weekly repeat, which is surprisingly a very common requirement.
Now you have a simple way to overcome Samsung's deficiency.