The error says that the configuration section is locked at the parent level. So it will not be directly 1 config file which will resolve the issue, we need to go through the hierarchy of the config files to see the inheritance Check the below link to go through the File hierarchy and inheritance in IIS
https://msdn.microsoft.com/en-us/library/ms178685.aspx
So you need to check for the app config settings in the below order
Go carefully through all these configs in the order of 1 to 6 and you should find it.