@Kaitlin61 when you set up a listing the 20% discount usually appears automatically so I wonder if you switched it off by mistake. You should be able to switch it on again but you'll have to trawl through the listing to find out where.
@Kaitlin61 I was having a similar issue with a second promotion that I was trying to run within the same month. The program would not allow me to select additional dates on the calendar within a week of the first promotion offer. I reported it to customer support but they were ineffective in resolving the issue and unresponsive to my queries for help. Were you able to get this issue resolved to run multiple promotions?