You do not need a prototype to validate a product idea. Remember that a business starts with a marketing strategy, then tests assumptions, and validates the strategy. Doing so does not require having a tangible prototype or or MVP. The validation for product-market fit defines what your product should actually be, to whom it will be sold, and by what method/manner. While some concepts are easier to demonstrate than describe, you should always be prepared to make radical adjustments to your assumptions before you commit to creating your product or service.
A prototype is better at helping you figure out whether the product you have discovered people will actually buy, is something that can really be produced, within the confines of what people are willing to pay, and how much money you need to operate and can spend on production. Your conceptual model only needs to be as detailed as required to get the insight you need to make further assessments. Sometimes a description is enough, sometimes you need to make one (mostly) working widget.
Having a prototype can accelerate the speed at which your research proceeds, but it is rarely essential. If your concern is speed, then try a prototype. If your concern is cost, then spend your time on your description.