Yes, I too have made a comment just now on being given an incorrect result for not putting in the definite article for translation of the german into English. I have also noted in the app that I don't need to put in the definite article but the online platform marks incorrect any translation without the definite article also included in front of the noun.
I have heard this word often enough in German street markets, in midlands anyway, it sounded like 'layder' as in 'jeder Leder zwanzig Euro' ( they were belts for holding pants up) but where I fail is in answering 'Das Silber' as 'silver' and Das Leder as 'leather'. I should have written 'that leather'. ( I know I am cluttering but having read all below, I am still in the dark so zu sagen)
Duo seems to be very inconsistent about the "correct" English translation for abstract or mass nouns. For example, the German "Die Natur" can be correctly translated as just "Nature". In the example here for materials such as leather, the only way an English speaker would use the definite pronoun would be in the context of a specific piece of leather, such as "Please bring me the red leather over on the table." But given just an isolated noun, "Das Leder", there are no clues to let an English speaker know that this phrase is any different from "Die Natur".