On iOS Evernote provides a useful facility to be able to protect your notes by adding a four digit passcode lock that you have to enter before you can see them. You can also choose how long before the code is asked for on subsequent opens, such as immediately, after one minute etc.
Once enabled this means that you will be initially be asked for your four digit code on opening and then on subsequent opens after the time out period has expired unless you have chosen “immediate” as your option in which case you will always be asked for your code on opening.
Evernote also has a neat facility that allows third party apps to request access to your evernote account via the app itself, meaning that you don’t have to enter your username and password to grant access as the app is already logged in. Except that this doesn’t work if you have passcode lock enabled.
When an app requests permissions it does so by transferring control to the Evernote app, if you have it enabled you need to enter your code and once you do so you no longer get the option to authorise the other app.
The only way around this is either to temporarily turn off pass code lock (which is a pain) or set the time out period to something like “1 minute” then open Evernote and enter your code and then go to the third party app and do the authorisation. Providing that you are within the time out period (one minute in this case) you will see the authorisation page and be able to proceed.