Stone Brick is required to make floors so you can properly build a Castle Room. This guide on How To Make Stone Brick In V Rising will tell you everything you need to know about Stone Bricks so you understand where you can find them, how you can craft them, and what you need to unlock in order to make them a resource.
One of the objectives, if you continue with the story, requires you to build flooring and walls so that you can create rooms in the castle. This step needs Stone Bricks in order to progress, as Stone Bricks are one of the requirements needed to make the flooring.
How To Make Stone Brick In V Rising
Follow the main story until you unlock the Grinder, this is a special building you can construct that allows you to grind down stones to get different items. You can use the Grinder to create Stone Bricks, allowing you to make flooring panels and progress with the main story.