@connor Have you directly asked the same permission
You should not use this permission until you want to Store anything in the Device of user as the permission is so much stronger then WRITE_EXTERNAL_STORAGE
And as the app built from Niotron or Mit app inventor they both lacks the MANAGE_EXTERNAL_STORAGE permission in there manifest and so you need to use an extension to add that permission in manifest of the app and you should Also promt the users that your app is accessing the storage and you are not using any personal information or anything and also promt the window where the user is enable the app to allow all files access
And one more thing for most work READ_EXTERNAL_STORAGE is enough
Here go through theses articles to know more :-