Why is Google Maps telling me that Offline Maps aren't available?
It turns out that just because you can see and use a map online, that doesn't mean that Google will allow you to make the area available offline.

Offline maps are only currently available for certain countries, and Brazil isn't currently one of them, in fact it seems to be a fairly odd selection.

Google have published the full list of countries that it's available in here: Maps for Mobile - Offline Availability



EDIT: note that in a July 2013 Google Maps update, Google made offline maps into a hidden feature with no menu option any more. More about how to make maps available offline here: Get offline maps in new Google Maps