Not able to add new value to lookup (List of schema fields for portal access) using System Admin account

Hi Team

Value of a lookup is not accessible to a portal user but accessible to a normal user.

I have updated operation permission 

I have updated object permission

The Section object (Request) is available in lookup 'List of objects available for portal users' 

But same is not available in lookup 'List of schema fields for portal access'

I am unable to add value to List of schema fields for portal access lookup as fields are locked but as per academy we can add new value to this lookup.

Do let me know any work around to solve this issue.

Thank You in Advance