Normally this means you have good coffee, i.e. the material, the beans or the final product. That is uncountable, you cannot use "a". You could use "a coffee" to refer to a cup of coffee, but would you use that in the above sentence?
If I had to translate this sentence from English to Czech, I would say: Máte dobré kávy. So how do you know when to use the nominative or accusative case?
Normally you use always accusative. Here the genitive you proposed would be clearly wrong.
You use genitive when coupled with words of quantity like "trochu", "mnoho", "hodně", "málo", where you often use "of" in English.
Then sometimes you can use the partitive genitive, especially in negative sentences, but that is an andvanced topic. The above mention cases actually belong into this category, but there is more where you can sometimes use genitive, but it may sound archaic and it is really advanced. For example "Namám kávy." is theoretically posible, but will sound archaic or just strange. Just use accusative.
does the adjective always follows the noun? for exemple why not saying donrá kávu instead of dobrou kávu?