Dunning is run through F150 Transactions .
You have to use the batch job to schedule the Dunning Program .
RF150SMS is the program that you may use in the SE38 to run it and SM36 to schedule it .
1 . RUN F150 program with desired parameters.
2. Using the RUN date and RUN ID of the F150 run create a variant for RF150SMS.
3.Schedule a Batch job for the RF150SMS with Variant Created in Step 2 , though SM36.
Thanks.