As it says "Google App", it's most likely the app named Google (no kidding: there's such one, just follow the link xe2x80x93 though I cannot find it in Settings xe2x80xba Apps by that name). Just tried the one from the drawer, brings me to Google Now xe2x80x93 which again matches the app you've got trouble with, as your question states: "wanted to access Google Now, but it wasn't there".
Going by the icon, the corresponding app in Settings must be Google Search xe2x80x93 which again makes sense considering Google Now is part of that.
So I'd suggest you try to go there (in Settings xe2x80xba Apps) and clear it's cache. If that doesn't help, try clearing its data as well (same place). If that doesn't solve it, open the Google Settings app (not the general device settings, but the settings app with the "g" for "Google" in its icon xe2x80x93 you can find it in the app drawer) and deactivate Now and Cards there. This disables Google Now, but solves the issue (as you've proven).
Hint in case you cannot reach out to Settings or the Google Settings app (e.g. if that pop-up reappears faster than you can tap somewhere else): You can clear all app caches at once by booting to recovery-mode and "wipe cache". No data lost that way. Another good hint in this context is safe-mode: if the troublemaker is a user-app, it won't be started here, so you can clear cache/delete data/deactivate/uninstall such an app from there.