The activated ability’s cost checks the land’s subtype, not its name. You can sacrifice a nonbasic land this way as long as it has the subtype Forest or Plains.

Sacrificing a Forest or Plains is part of the cost of Knight of the Reliquary’s activated ability. Assuming that sacrificing the land puts it into your graveyard, Knight of the Reliquary’s first ability will immediately give it an additional +1/+1 when that cost is paid because there’ll be a new land card in your graveyard. Paying a cost can’t be responded to (with Shock, for example).

Knight of the Reliquary’s activated ability lets you find any land card, not just a basic land card.