Bug Player Falling through Procedurally generated terrain even though terrain has a mesh collider.

Hello, I am new to c# and am just starting unity.
I followed Sebastian Lague's tutorial on how to code Procedural Map Generation. I am having problems with the terrain's collider. When I use a fps controller to move around, I can move around for a certain amount of time and then I fall straight through the terrain. When i start running, I fall through the terrain and when I walk on mountains i fall through. When I move a certain distance, I fall. When I walk through water, I fall.
I have tried everything from changing my fps controller to trying to add a plane at Y = 0 to stop me from falling. Nothing has worked and I am very confused

The player Character has a mesh collider and has a rigid body, The terrain also has a mesh collider. Still I fall.

Someone Please help, and I would always be in dept to the person who helps.
Thank you so much