RTS pathfinding?

I’m 12 years old and I’m making a RTS game. I sculpt the terrain.

You may be able to get by using Blender’s built in nav-mesh system or a simple implementation of A* if you keep your game relatively simple and don’t have too many units moving around. However, if you’re aiming for something more complex and want to handle upwards of 100 units moving around at the same time, you’re going to need something much more complex and robust.

If you’re serious about your project, I highly suggest you do a bit of reading first. This site is probably a good place to start.

Or this site may be a little more easy to digest at first:
A star for beginners

However, using the built in system in blender is the best choice unless you are generating random terrain. The built in pathfinding is likely to be much faster that any python A* system even with a lot of units.

I can do a* pathfinding but how do I detect walkable terrain?

You have to either mark the nodes yourself or make a system that will mark them for you (based on things like face slopes, object properties, location, etc.)