To set the folder or file permission in Plesk 17.X, please follow the following steps :-
1. Log into Plesk Panel.
2. Click on the "File Manager" Icon under your domain.
3. Navigate to the file or directory for which you want to set access permissions, and click the "arrow" icon in the corresponding row
4. On the right hand side of the folder or file Click "Change Permissions" in the drop-down menu.
5. Click on the appropriate User from left e.g. (Application pool group (IWPG_(your_domain_user_name)) next adjust the permissions as required on the right side then click "Ok" or "APPLY" to save.
Once saved the settings reload site to confirm your site or application is now working as expected.