First, a spelling pass (yeah, I’m THAT guy, sorry):

“A former [m]arine[‘]s first day at Walmart is turned upside down when eco-terrorist[s] capture the store and take his sister hostage.”

This LL touches all the bases with no excess blather (almost). Protag, antag, backstory, conflict, stakes, setting. Good job. Some suggestions:

– Is the marine doing security at Walmart or a regular-Joe job, like checkout operator? The “gap” (difference between what the protag expects to happen and what actually happens) is wider if he’s doing a mundane job unrelated to his former career.

– “is turned upside down” is unwanted filler. Let’s toss it and flip the focus onto the protag/marine to let him drive the story forward, instead of being acted upon:

“On his first day at Walmarts, a former marine must save his sister when eco-terrorists seize the store and take hostages.”

That hits all your notes and avoids the fluff of “turned upside down”. But honestly, your LL does the job already, Richiev. Go for it. Write what you know (Hooah!) and write what you don’t know: what would I need to do to save my sister if terrorists stormed my workplace right now?)