That's what I said (the App store download, not the pop-up). Your Facebook account should have nothing to do with the App store. Your Apple ID should be an email address. If you used the same email to sign up for Facebook, that's irrelevant. If you need to change the info for your Apple ID, log in with it here: https://appleid.apple.com/#!&page=signin
You do know your Apple ID and password, right? If not, follow either the "Find your Apple ID" or "Forgot password?" links on the page I previously linked you to: https://support.apple.com/apple-id
You're not going to be able to download anything from the App Store, free or not, unless you're signed in with your Apple ID, so you need to get that straightened out.
And the popups probably won't go away even if you install Flash from Adobe. You don't need Flash, unless you watch videos that require it (most don't anymore). But scammers have been using the Flash popup for many years to get you to go to their website So, get the Apple ID sorted out, get Ghostery, turn it on to block everything and the popups should go away.
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