The "Community" research tree contains a number of public order and research rate bonuses which can be useful to all factions. To unlock the tree, you will need to either upgrade a "monastery" (or unique variant) or conquer one at max level.

If you're playing as one of the factions based in England or Wales, you probably won't find any monasteries even in the very late game - the chain just stays frustratingly locked.

In order to unlock this tree, you will need to conquer a provincial capital where the main settlement building is in the "Monastery" chain. No amount of mucking around with regular churches or other unique religious buildings will help (even though some of them are called monasteries - e.g. the Monastery of St Edmund). To unlock the tech, it needs to be the main settlement building itself.

There are only seven monasteries in the game - 2 in Scotland and 5 in Ireland.

The locations are:

Scotland:
Dun Cailden (in the middle of the highlands) - initially owned by Atfochla
Ioua (in the hebrides) - initially owned by Sudreyar

Ireland:
Cluain Mac Nois (bang in the centre) - initially owned by Mide
Archhadh Bo (central - just south of Cluain Mac Nois) - initially owned by Ospraige
Dun Patraic (on the east coast) - initially owned by Ulaid
Ard Macha (in the north east - just inland from Dun Patraic) - initially owned by Airgialla
Inis Faithlenn (southwest coast) - initially owned by Iarmamu

If you're based in England, the easiest to get to is probably Dun Patraic.

The Monasteries in Dun Cailden, Cluain Mac Nois and Ard Macha are each dedicated to a particular saint and they give a fame bonus and some extra buffs.

Hope this helps.