Yes, there are around 40 countries missing from the Good Country Index (including Taiwan, Cuba, Bhutan, Turkmenistan, Tajikistan, Eritrea and several other beautiful, ancient, much-loved and highly respectable nations).
There is only ever one reason for this: because we don't have enough data to give them a reliable ranking. And if the United Nations and the other organisations don't collect or don't publish the data, there's nothing we can do about it except wait until they do.
It's NOT because we've accidentally left them out, or we've never heard of them, or don't consider them a proper country, or because we're afraid of offending some other country that doesn't consider them a proper country, or because their scores are too low.
All countries with missing data on more than 2 out of 5 indicators on any category are excluded. The two exceptions to this rule in the first edition (1.0) of the Good Country Index are Iceland and Georgia, which both have 3 missing values on the “Planet and Climate” category, but only 4 and 5 missing values overall.
Since the rankings are based on mean scores per category and missing values are ignored, the countries included in the Index are neither rewarded nor punished for any non-reporting.