Can you provide a full screenshot of the problem? There can be a few reasons why something might go wrong
The reason why it shows nan in this case is because your propellant is too weak to actually propel the bullet at a meaningful speed.
Some things I noted from a quick lookover:
1. Projectile to casing ratio looks to be off. Generally when you are starting out you always want your projectile to be shorter than your casing.
2. M26 is the most reliable propellant to use as RLG can get pretty crazy if you don't experiment with it significantly. With that, your propellant grain is also very massive for a small arms cartridge. You want many small propellant grains for a faster burn, ex 1mm or sub 1mm grains. You can visualize this by seeing how different shapes burn in real life!
You also may want to play with perforated grains if desired performance is not met due to perforated grains being able to burn from both the outside and inside at the same time leading to a neutral burn time graph.
Hopefully this helps!