You can edit the materials if you make a mistake while setting up the list. The materials list should contain all possible materials you may be using in the simulation. You are not, at this stage, specifying which materials will be used. You can specify which materials apply to which parts of the simulated domain in the Cell Zone Conditions.

In a simulation where there is only one material, this may seem like an unnecessary step. However, in a simulation with multiple materials (for example mixing or a chemical reaction), you want to be able to specify different materials (or mixture of different materials) in different zones. If you have only one material in your simulation, if that material exists in the list, and the material properties are correct, you can proceed to ensure it is selected in the Cell Zone conditions.

If you want to add more materials to the list, you can edit one of the existing materials and click No when asked to overwrite.