fantia is a subscription system, you can only download content during the subscription period. (；′⌒`)
You can get version 0.79 here.
【ゲーム】 【更新】シャハタ遭難の一日 (Beta ver0.79a) - JaShinn Game (JaShinn)の投稿｜ファンティア[Fantia]
Or you can wait for future steam and DLsite versions to be released.