When you sign in, 1krNote asks Google Play (on Android) or Apple (on iPhone) to confirm that it's the genuine app from the store, on a phone that hasn't been tampered with. If that fails you see "This copy of 1krNote couldn't be verified. Install it from Google Play." and you can't sign in on that phone. Notes and reminders keep working without an account.
On Android:
- Install 1krNote from Google Play. Copies from other places can't be verified.
- Update Google Play services and the Play Store app; the phone must be signed in to a Google account.
- Rooted phones or custom Android versions usually can't pass Google Play's check.
On iPhone: install 1krNote from the App Store and keep iOS up to date. Test copies (for example installed with a sideloading tool) can't be verified.
If it still fails with the store version on an unmodified phone, contact support with your phone model and system version.