Does not name a type

Now I’m assuming you wrote the code exactly as specified:

struct node *nodes[1024];
nodes[1]->data = 1;
nodes[1]->left = NULL;
nodes[1]->right = NULL;

The reason you are getting compiler errors is because that is not valid C++ code.

But if you move that code into a function it will compile just fine:

struct node *nodes[1024];

void AddFunction()
{
  nodes[1]->data = 1;
  nodes[1]->left = NULL;
  nodes[1]->right = NULL;
}