not everything is correct,
first, save your file as a .c file(download the the update, and it will stop saving as wdl.)
second your function:
function quit game(); // The function that quits the game.
{
while (key_any) {wait(1);}
sys_exit (NULL);
}
->fınction names cant have spaces. fix that, and since this is not a protype, remove the ";" after paranthesis.
last one:
flags OVERLAY | VISIBLE -> you forgot a = and a ; here.
flags = OVERLAY | VISIBLE ;