(If you hadn’t posted I would’ve backed out of it too.)
On desktop just press the X and on mobile click Not Now
OP is sharing a screenshot of a login form.
On mobile, at least, Dropbox inserts their own interstitial page to download the app instead of immediately displaying the file, which may be what the parent comment is referring to. At a quick glance it might look like a login wall since you're obviously expecting to see an image instead.