The “Missions” feature is only included in the Android app.
One option: on the website, head to your Lists, then Life List. You can then search for your region, and click “Unobserved Species”. This will give you a somewhat similar list of “missions” as the Android app. e.g. https://www.inaturalist.org/lifelists/bouteloua?view=tree&details_view=unobservedSpecies&tree_mode=full_taxonomy&place_id=1859
There are a couple other (less user friendly) search options listed here: https://forum.inaturalist.org/t/missions-feature-on-the-website/6121/4