Before getting the answer, please read the following things.
How can I enable a feature on a plan and disable it on another plan?
- Go to Listing Plans -> Your Plan
- Select enable / disable status
- Finally, click Save Changes or Publish button

How can I hide disable feature on Add Listing page?
- Go to Appearance -> Theme Options
- Navigate to Add Listing General Settings
- Set It should not show on Add Listing page option to Unchecked features will

So, if you see a grey field on your Add Listing page, which means:
- This field is disabled on the current Listing Plan that you are using
- You are using Show on Add Listing page, but it will be disabled option mode on Theme Options setting
To resolve it:
- You can re-enable this feature on the Listing Plan
- Or select the second option on the Theme Options