This is currently unsupported. See:
- [Feature request] Password or pin protection · Issue #289 · laurent22/joplin · GitHub
- Password in Android APP and Windows program
- Access PIN (Will PAY for feature$$$)
- Touch ID and PIN code protection for all apps
If you require a security measure like this on Android:
- Use one of multiple different secure notebook apps that support this feature
- Use an app locker
- Use an app isolator
- Contract someone to make a private build for you
- Develop it yourself