Translation:If they were to walk, they would see the beautiful environment
Native speaker of (American) English here. If the singular "environment" is used then an article is required: ... see a/the beautiful environment. However, if the plural word "surroundings" is used, then you have a choice of no article or definite article: -- see beautiful surroundings / see the beautiful surroundings. The second choice (with the definite article) still sounds better to me.