When you install Portal for ArcGIS, the account that runs the Portal for ArcGIS service (known as the Portal for ArcGIS account) is configured to run under the Windows native LocalSystem account by default. The LocalSystem account is not intended for accessing network locations; therefore, you cannot specify a shared network location as your portal's content directory.
If you want to configure the Portal for ArcGIS account to run under a different account, such as a local or domain user, you can do so after the installation has completed. If the account you specify has the appropriate permissions, you can also move your portal's content directory from a local directory to a shared network location.
To learn more, see Changing the Portal for ArcGIS account.