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 ;