"Is angels able to love vampires?" was accepted as a correct answer. I put: "Are angels able to love vampires?" Help, anybody?
I'm not sure what you are asking. I'm going to explain this like you are not a native English speaker. Sorry if I'm mistaken.
"Is angels..." is clearly wrong because the subject "angels" is plural and "is" is singular. "Are angels..." is perfectly acceptable English.
If Duo accepted "Is angels..." and not "Are angels" that's a screw up on Duo's part, you're good.
When in doubt, go to the Oxford English Dictionary. "Engel" is in it. It was an alternative spelling of "Angel".