What browser are you using? Can you take a screenshot of that? Also, you could try the troubleshooting link in my signature line.
Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
I don't see anything on BYC's end that should prevent you. Can you tell us what happens when you try to post an ad? Can you click on "Create a new classified" and get a template for creating the ad?
Also, here are the FAQ's for BST; perhaps something there will help:
https://www.backyardchickens.com/a/buy-sell-trade-bst-faqs