Sepulchrum is neuter. The plural form here is -a, as is the case fall all neuters. Think bacterium and bacteria.
Could Duolingo stop using the yellow color for new words it makes them difficult to read. How about blue or green. I thought it read Libi instead of Ubi
I didn't even remember that I knew the word "sepulchre" in English, but I knew what "sepulchrum" was pretty much straight away. Neat