After 20 years, what were the lessons learned from Afghanistan?

Same lessons we never learn:
Nation building doesn't work.
You can't buy determination and leadership.
The policeman of the world policy went sour a long time ago and the USA didn't adjust. Unfortunately China has a policy of buying into a country and never leaving. Works for them and will work against us long term.
Endgame and exit strategy are more important long term.

The U.S military can't make a government legitimate unless we remain for multiple generations. I was in Kandahar during the Obama surge 2010 - 2011 and there is no cultural knowledge overlap between the U.S. and any Coalition nation being able to relate to the typical Afghan citizen.
NATO / Germany, Japan and Korea are examples of long-term multi-generational military involvement and investment. I remember Red brigade terrorist attacks in all of these countries prior to the fall of the Berlin Wall in 1989. World War 2 had the U.S. as military governships for much of Western Europe, Japan and for South Korea, our involvement in the Korean War. There was no guarantee of cultural and military success in all of these countries we view as stable democratic countries now.
The U.S. must decide what is in our interests and what level of involvement we need to support these interests. Central Asia / Afghanistan was not a natural sphere of influence for the U.S. It has been either Russia, British, then India or China. We should encourage countries closer to Afghanistan to invest and maintain a presence for a longer term relationship.
Our involvement in Vietnam, Central and South America, parts of Asia and Africa that are not multi-generational and temporary in nature, do not last and they are not effective.
Once the Berlin Wall fell the risk of NATO or the U.S. falling to a communist power went away.
We continue to insert ourselves into the affairs of many countries without a consistent long-term strategy, let's hope we learn from our Afghanistan involvement.

Technology doesn't always win. Great Britain had the technology and lost. The Soviets had the technology advantage and got routed. Enter the US with all our great technology. Once again, Afghanistan wins. We should have learned that lesson in Vietnam but didn't