Both the mass noun and the plural form are possible in Finnish. We chose the mass noun as the primary translation since it's closer to the American English expression. The expression olla loppu, literally "to be the end", means that there is nothing left. There is no need for aivan, or kokonaan, or any other word to be added, unless you add a word also in the English sentence. Another way to translate this sentence is to use the expression "to be out of", as in "I am/You are/We are/etc. out of candy". :)
I disagree. During my 40+ years as Finnish-speaking I have not once heard that karkki would be used as a mass noun. If there are more than one sweet it is used only in plural. Edit: This appears to be more complicated than I first thought. My grammatical skills are not enough to create a rule for using karkki. But IMHO, in a sentence like this one, karkki is far more often in plural: Voi ei! Karkit on loppu.
I searched for candy related articles on YLE and the results are a mixed bag (ahem). The general rule is that if you are talking about a specific type of sweets, the countable version is preferred, whereas unspecified sweets are more likely to be uncountable. Despite these preferences, both the countable and the uncountable options are possible in both cases. The more formal makeinen, however, is always countable. Here is a pretty typical article on sweets. As you can see, both are used and the uncountable version even appears in the title. :)