According to the OLD, there is a plural word altāria, altārium , n. pl., meaning "an altar-fitting for burnt offerings (often not distinguishable from āra )"; also "a detachable and portable" form of an altar.
The singular form of altāria would be altāre . But the singular forms are rare and late; the plural forms are used with a singular meaning.
āra f (genitive ārae); first declension, "altar". From āsa, ultimately from Proto-Indo-European *h₂eHs- "to become dry; to burn". Related to English arid, and Spanish arder ("to burn") and asar ("to roast"). Also related to star, astral and stellar, English words of Germanic, Greek and Latin origins. Other cognates include Ancient Greek: ἄζω (ázō, “to dry”), Hittite ḫāššā- (“fireplace, hearth”), Sanskrit आस (ā́sa, “ashes, dust”), Tocharian B astare (“pure”), Welsh odyn (“kiln”), and, last but not least, English arson and ash.
I feel like this course takes a very different approach to Latin than most school courses. Latin breaks all of it's own rules, and interpreting text to match a key is not always simple, even if you do know all of the rules. That being said, I only had one issue so far, so the programmers have been doing a good job allowing for differences in linguistic order.