The handbook is just for stuff that wasn't on the wiki already, since we're planning on building a new, complete documentation from scratch. The link I provided has all the info.
Overloards for NPC.spawn are:
NPC.spawn(id, x, y, section)
NPC.spawn(id, x, y, section, shouldRespawn?)
NPC.spawn(id, x, y, section, shouldRespawn?, centered?)
The types of each argument are (number, number, number, number, boolean, boolean)
In your line you put NPC.spawn(number, number, number, number, number)
So it complains because it expected true or false for "shouldRespawn?".
ShouldRespawn and centered both default to "false".
Removing the ", 0" should fix it.
That said, you're unlikely to see any changes afterwards. Since the coordinates and section you spawn at seem kinda suspicious. You're spawning at coordinates 67,69 in section 1 (the 2nd section). That's usually where section 10 is situated. Here's an image of how the sections in SMBX are laid out:
(not entirely accurate since section 10's top left corner is 0,0)
To spawn the fuzzy on top of the player, you can spawn it at the coordinates player.x, player.y.
To ensure you spawn it in the player's section, you can spawn it in the section player.section.
So your call would look like this:
NPC.spawn(999, player.x, player.y, player.section)