To use the advanced search page, you must use a browser in desktop mode, not the eBay app.
What are you trying to do, exactly? Many of the features on the advanced search page are available in the eBay app, but in different locations. Try doing a search, and then checking the available options under the filter menu to start with.
Or create a search using a desktop browser, then save the search. You can access your saved searches from within the eBay app.
