Here's why:
* Sentence: A sentence is a grammatical unit that expresses a complete thought. It's a singular entity.
* Scenery: Scenery refers to the natural features of a landscape. It is usually a collective noun, but in this case, it's used incorrectly.
* Beautiful: An adjective describing the scenery.
To make a plural, we need a noun that can be pluralized.
For example, if you wanted to say "beautiful scenery," the plural form would be "beautiful scenes" or "beautiful landscapes".