So you can cheat the Test Case editor by casting a TestObject returned by findTestObject(...) to a RequestObject.
Please note, this underline is not very significant. It is just a warning emitted by the editor. It is possibly not the reason why you test case fails. Even if the underline is erased, you test case may still fail due to some reason which is not yet looked at.
I mean, we need the ObjectRepository to implement findRequestObject method which returns an instance of RequestObject. With it, no type-casting will be required.
Unfortunately findRequestObject method is not provided actually.
This tells nothing to others.
Please provide more information: logs, screenshot, your test case script, test object definition, the target URL if it is public etc. See the following guidance.