Enabling parent paths specifies whether an ASP page allows paths relative to the current directory (using the ..\ notation).
If set to true, this property constitutes a potential security risk, because an include path could access critical or confidential files outside the root directory of the application.

In IIS 6.0, parent paths are no longer enabled by default.
This affects your application if it has a Web page that contains the #include server-side include directive and uses “..” notation to refer to a parent directory.

Steps to enable parent path in PLESK:

1. Login to PLESK control panel

2. Click Home

3. Under Hosting >> click Web directories >> Preferences >> Application Settings>> Enable parenth path >> click ok

Now parent path is enabled for your domain.