Open Your linked app from linkedIn developer account. Then Navigate to Mobile Menu: Please check screenshot below
Please check if you have Application ID : <01234> whatever, used in your IOS app info.plist.
Also confirm if you have added right IOS Bundle Identifiers: by tapping on ADD Button, and if needed IOS URL Scheme Suffixes in LinkedIn App.
Tap on Update Button and then try Login with linkedin on app, it will work.
-Happy Coding.