I’m using a “appodeal” plugin and it contains “WRITE_EXTERNAL_STORAGE” permissions.
However, when I run the app first time, the permission request popup(like “allow access to… blah blah…”) does not appear.
Is this normal? I think it’s a bug.
How do I get a permission request popup appear?
If I’m understanding your question correctly, then that permission is given when the user downloads the app from Google Play (or from other instances).
Whatever permissions you add to your build.settings are shown and need to be accepted by the user at the moment of download/install, so by the time the app is run for the first time, that permission has already been given. I don’t think there is any way to later deny that permission either.
Thank you for your detailed answer!!! 
I thought the question was solved. (Checked on test devices running Android 5.0.1)
But
On Android 6.0 and higher test devices, the permission agreement popup does not appear when installing the game.
Does anyone know of this bug?
I received an email from a Korean public agency
asking me to add a popup (at installation or app launch).
They said that if I didn’t fix, they could fine me according to the law. 
What exactly does your app do that requires a government agency to threaten you with fines? I can’t comprehend how saving files to an SD card could result in fines.
You can look at https://docs.coronalabs.com/native/android/permissionSupport.html#about-android-runtime-permissions
Alternatively, you can check to see if you have access to the remote storage (https://docs.coronalabs.com/api/library/system/getInfo.html#android). I don’t think that even if the permission is listed in the build.settings that it is on by default, so if and when getInfo() tells you that it isn’t active, you create a custom pop (such as a black rectangle that covers the screen and white text) that tells the user to go activate it from the app settings.
Now, I think that Appodeal’s external storage needs might fall under something that does not require a specific permission (see https://developer.android.com/training/data-storage/files/external-scoped). Appodeal is most likely just caching ads and it doesn’t require access to whatever else files you may have on your external storage. Again, I’d want to know how, why or what some Korean government agency is trying to fine you for.
Are they requesting the fine be paid via crypto currency? I suspect that is just a spam e-mail.
appreciative of your comment.
Because my place of business is in South Korea.
and I checked the issue of Appodeal banner ads not appearing without “WRITE_EXTERNAL_STORAGE”
now I’m thinking about creating a custom popup as your advice.