Emacs recognizes 5 modifier keys (arguably 6 depending on how you feel about Shift):
<Control> (C-), <Meta> (M-), <Super> (s-), <Hyper> (H-), and <Alt> (A-).
This capital A means that this key-sequence uses the uncommon <Alt> modifier. Since your Alt key, if you even have one, is most likely used for the <Meta> modifier, you probably do not have a key currently mapped to work as this modifier. However, you can simulate its use with the key sequence C-x @ a. In this case, C-x @ a M-n is equivalent to A-M-n. (The similar key sequences C-x @ h and C-x @ s are equivalent to using the <Hyper> and <Super> modifiers, respectively.)
It's worth noting that it is possible to rebind modifier keys to work as different modifiers. For example, on Windows, the Alt key can be rebound to work as the <Alt> modifier by setting the variable w32-alt-is-meta to a nil value.