Because alle counts as a declined article, so it requires the weak declension, and therefore the plural adjectives get -n. See https://www.germanveryeasy.com/m/adjective-declension for the list.
Just my opinion, but I think the fastest way to learn German is to concentrate on vocabulary, word order and grammar last of all; it will gradually fall into place as you practice, practice , practice. In my view, one learns best by making mistakes and having them corrected.
I answered this as "All possible..." having got used to wierd Duo sentences. But I had a look through my dictionaries and struggle to make sense of the use of 'möglich' to mean 'all sorts of' Surely 'möglich' is 'possible' or 'potential' and 'all sots of' should be translated as one of 'allerlei' vielerlei' or 'allerart'