It means the file containing main doesn't have access to the player structure definition (i.e. doesn't know what it looks like).
Try including it in header.h or make a constructor-like function that allocates it if it's to be an opaque object.
If your goal is to hide the implementation of the structure, do this in a C file that has access to the struct:
struct player *
init_player(...)
{
struct player *p = calloc(1, sizeof *p);
/* ... */
return p;
}
However if the implementation shouldn't be hidden - i.e. main should legally say p->canPlay = 1 it would be better to put the definition of the structure in header.h.