Wrong on all counts, I'm afraid.
Grand... occurs in all the Romance languages, and of course English. See https://en.wiktionary.org/wiki/grand#Etymology. It is not Greek, whose word for grande is μεγάλος (say "megàlos").
The e/i ending is not Greek nor irregular, it is just another regular form of inflection. See https://en.wikipedia.org/wiki/Italian_grammar#Inflection_of_nouns_and_adjectives.
Capacità is neither Greek (Latin capacitas) nor irregular. The regular rule is that all nouns ending in a stressed vowel are invariant in singular and plural.
Valm 86 is right. If you translate 'capacità' to 'capacities' then the sentence translates to 'You (all) have great capacities' . Clearly if you use Avete then grandi is correct. Had hai been used instead then it would all be singular 'You have great capacity ' and grande would be used.
Do you mean that Duo marks them incorrect or that in your personal opinion they are incorrect?
I played safe and (because of the plural) put "great capacities", a literal translation that I'd never normally use. Marked correct, but ridiculous.
Dictionary examples suggest that capacità is best translated to capability/ies or ability/ies, the former leaning to a mental context and the latter to a physical context.
Potential is an inaccurate translation of capacità; it matches il potenziale.
Holding capacity (volume, stadium seating, etc.) is la capienza
Note the plural adjective. Try cap/abilitIES. Duo's use of "potential" seems to be either poor translation or Italian usage, i.e. several abilities = great potential. I can't find a dictionary that supports it, potenziale being the standard word and in Collins "to have potential" = essere promettente.
In English,"capability" and "potential" are different: the first means developed skills (e.g. Clark Kent working at the Daily Planet), whereas the second suggests the strong possibility of the skill being developed (e.g. Clark Kent as a toddler).
Is the Italian term ambiguous between these two interpretations?