why can't I customize this link? This is the error I am getting
That's not an error. It's the way the URL Builder feature works. It lets you design URLs with certain query params pre-set so you can paste those URLs into other services, in emails, etc. That doesn't change the core URL of the LP so there's nothing to save.
If you want to change the core URL, that's Edit URL Settings in the same menu.
Normally It looks like this and I am able to customize- the image I showed you before does not let me edit:
Simple: the Available Fields correspond to hidden fields on a named form on the LP. No hidden fields (or no form at all), no Available Fields to build with.
The assumption -- correct or not -- is that the only query params you deem interesting are those that are known upfront to map to Marketo fields. This makes the feature useless if you use the Forms JS API to add fields to forms on-the-fly. But again, it's always worked this way.
(And you're still editing a temporary representation of the URL for cutting and pasting, not changing the underlying page resource.)
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