EDIT: This question was originally marked as duplicate of 'How to respond when you are asked for an estimate?' I read over the answers, but I think my situation may be a little different, so I am completely rephrasing my question and elaborating on the context that I believe makes my situation a little bit different.
The answer above assumes that there is some kind of project scope, or an idea of what requirements there will be - and if not, at least some assumptions.
But my team is still in the discovery phase. We have not even officially kicked off on our project. We are still defining the problem statement, the user journey, and the experience roadmap for the project. So this means no defined scope, and no requirements yet.
The only assumptions I can make are that we're going to need to pull raw data from somewhere. My team lead has provided me with an excel sheet, and while I am perfectly comfortable working with that to provide a proof of concept, it is not sustainable as the primary source of information for the application because it is manually maintained. (And one of the ideas behind our project is that the user should be able to see the most up-to-date information available.)
He continued to ask about, "What are the steps needed to get to [that] point?" So I tried to get him to explain what [that] was. Instead of directly answering the question, he brought up an API that I had said we would probably need in order to communicate with some 3rd party software (as an example, I'm assuming). In the end, he was asking how long it would take to parse the excel sheet and JSON encode it - probably a couple of hours. He continued to ask 'what are the steps needed after that to get to a working prototype' - and I just don't know. There is literally nothing to go off of. I am working with the team to define a list of requirements, so I can get a better estimate, and we've determined that we can do that by next Wednesday, but my team lead wants the estimate tomorrow.
I could just go off of what my ideas for the application would be, and mark them as my assumptions, but at that point, if I start developing that, it could be a lot of time wasted, when I am simply asking for 4 days to work with the team as a whole. Not to mention - I believe designing with intent (and with the user in mind) is far more important than coming up with a list of ideas / assumptions about the app based on what kind of data is currently at my disposal.
What are my options, and what can I say or do to let him know that I can't estimate that based on nothing - or what am I supposed to do as an alternative? I don't want this to hinder the progress of the team, but we haven't even officially 'kicked off' yet - so I'm thinking this is rushed, premature, and eventually could be harmful to the overall development cycle and user experience. But I'm also new to this idea of 'professionalism' - so I want to try to be as clear as possible when I try to explain my [valid?] point?
Thank you, and hopefully this clears up my question.