I am making a first person shooter. I am making assets in Blender and importing them to Unity. I made a building with an entrance and imported it. When I walked around it, the walls started disappearing. To solve this,I re-calculated the normals, but then when I walked inside the building the walls disappeared. When I flipped the normals, I could only see the walls inside and not outside. Is there a way I can have the normals visible from the inside and outside of the model?
You should simply add thickness to your walls, as they said in the comments, you don't have walls made only by a plane in reality, so the fastest way you can adjust your model is by using the solidify modifier just add as much thickness as you want and you should have something similar to the first picture; as you can see, this way you have normals on every side. In the second picture you can see where the thickness value is and all the other values you can play with in the solidify modifier.
Difference between solidfy and no solidify:
For walls, you should add thickness as has been said. Sometimes you do need a paper-thin object, in which case you can simply duplicate the faces (SHIFT+D) and flip the normals ( W, F). Leave this for the last stage of modelling, because overlapping vertices are not edit-friendly.