@simgirl542 Hi, I am not sure if you already doing all these steps in order, but here goes some screenies anyways :)
Step 1
green....click on your sim's inventory
red.... click on knitted item
blue.... list on plopsy
https://i.imgur.com/uKQt3B6.png
Step2
Wait for your sim to complete her last interaction to list item on plopsy, it will be in the cue of things to do as you can see at red circle, unfortunately it does not happen instantly
https://i.imgur.com/8RXL1Ap.png
Step 3
Once she has done it the item will go pink in colour and is successfully listed on plopsy :)
https://i.imgur.com/B1D8dgh.png