Rule Definition
Using LAContext evaluatePolicy: method provides a callback reply block reply:(void (^)(BOOL success, NSError *error))reply. This block cannot stay empty, otherwise you won't be able to ensure that the user has been properly identified.

Remediation
Make sure you have implemented the reply block callback

Violation Code Sample

LAContext *myContext = [[LAContext alloc] init];
NSError *authError = nil;
NSString *myLocalizedReasonString = <#String explaining why app needs authentication#>;

if ([myContext canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&authError]) {
    [myContext evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
                  localizedReason:myLocalizedReasonString
                            reply:^(BOOL success, NSError *error) {
            
           // empty callback block
        }];
}
} else {
    // Could not evaluate policy; look at authError and present an appropriate message to user
}

Fixed Code Sample

LAContext *myContext = [[LAContext alloc] init];
NSError *authError = nil;
NSString *myLocalizedReasonString = <#String explaining why app needs authentication#>;
 
if ([myContext canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&authError]) {
    [myContext evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
                  localizedReason:myLocalizedReasonString
                            reply:^(BOOL success, NSError *error) {
            if (success) {
                // User authenticated successfully, take appropriate action
            } else {
                // User did not authenticate successfully, look at error and take appropriate action
            }
        }];
} else {
    // Could not evaluate policy; look at authError and present an appropriate message to user
}

https://developer.apple.com/library/ios/documentation/LocalAuthentication/Reference/LocalAuthentication_Framework/