Requirements are the foundation that you will use to design, and then build a solution for whatever business needs or goals are to be accomplished.
An Inception Deck is nothing more than a requirements elicitation technique that you can use at the begging of your project. A set of activities to engage stakeholders and draw out details about the project.
Ideally, requirements need to be complete, but in most cases they are not. Simply because when people have an idea or a need to meet, they can't imagine everything in details about it, or anticipate every potential issue or outcome.
So as part of business analysis activity, requirements are elicited using various techniques. To take something from a very broad idea or project statement, to something more detailed, and more focused, that can then be used to build a solution and set proper expectations going forward.
But using a technique like the Inception Deck doesn't mean that you've suddenly cast a magic spell on stakeholders and all of a sudden they know what they want, they know how to define it, and to clearly and precisely express their needs. That's not going to happen.
In fact, in practice it was observed that people are better at recognizing something that they don't want, as opposed to defining something they do want.
So because of this, any requirements elicitation, no matter if at the beginning of the project or otherwise, will have to be done in multiple passes, iteratively, and incrementally, in collaboration and communication with stakeholders.
And for example, one technique that helps even more in bringing focus onto the final solution you need to build is prototyping => you refine some of the need to have enough details to build a solution that you then put in front of the stakeholders and use it to elicit more information. Rinse and repeat until you get the final product out, or at least something that is good enough to use that requires no more iterations while you spend your time focusing on other ideas.
So it's fine if people say "I don't know", or "what do you think?", or "not quite what I imagined", because it means another iteration to refine the idea or the need, and to design a better solution for it. And if this continues for many iterations then that's fine too, because it allows you to identify a potential risk for your project from the get go. If there is so much back and forth when defining the project charter, then maybe there wasn't a strong need to begin with, and maybe there is no point in continuing with this project.