The honorific form of address and separate accentuations, for instance, are separated by commas in German. http://www.duden.de/sprachwissen/rechtschreibregeln/komma#K132
Basically you can call your own grandfather whatever cute name you want, but for Duolingo stick to using the literal term "Grandfather" for Großvater. On questions that use the familiar term Opa, feel free to suggest whatever you think are commonly-used familiar terms in English.
(P.S. "Is it wrong?" in English would be Ist es falsch? auf Deutsch, and in both languages the space goes after the comma, not before.)