the US has low scores in "Functioning of Government" (6.43) and also in "Political Culture" (6.25), but I don't understand those terms.
Lack of Compromise Politics.
This is what I think the "political culture" index reflects.
Normal politics has always depended on trading off what one side wants againsts what others want. It absolutely depends on compromise to work. Without compromise you have political deadlocks and chaos.
In the US it seems that far too many issues, big and small, are now pursued along bitter hard line approaches. Compromises are begrudging at best and absent at worst.
In practical politics we need politicians to be pragmatic about getting things done. The fact that a solution is not the ideal one you want should not be bar to making an agreement. One piece of a pie is better than no pie at all.
Politicians in the US (although not only the US) are increasingly adopting hard line stances and engaging in political retoric which attract and entrenches extreme positions. In a healthy political culture a politician always leaves themselves room for compromise because they know eventually that achieving any practical goal will need compromise. In the US politicians are now adopting positions where they have no wiggle room.
Politicians are, I think, afraid to express the view that, yes, compromise is a necessary and a good thing.
This is fear is amplified by a media which feeds off the polarization of every political issue. Politicians who fail to meet the unreasonable expectation of not giving an inch get lacerated by the media.
Functioning of Government.
The US has had trouble even agreeing a budget. When you cannot pay your own civil servants, not for lack of funds, but because you cannot agree to write the check, it's bad. That's a pretty awful state of play for a nation that identifies itself as a Capitalist state. That and the invasion of the seat of US government by a mob makes it clear the US is barely hanging on to a healthy political culture, if it is still hanging on at all.
A significant issue with a functioning government is that you ideally want one that works even when the politicians don't. That what a Civil Service is for. They keep the wheels in motion. A significant issue in the US is that there's a lot of political levers inside what should be the Civil Service. Where most countries have permanently employed officials, you have people who are elected or emplaced by political decisions. This makes the apparatus of government vulnerable to political exploitation in more direct way than the better democracies do.
You cannot have a functioning government when you politically interfere with it all the time. You have to let the train run on the tracks. You can't keep applying brake and accelerator together all the time. You cannot exclusively appoint people based on their political alignment and expect that to work because it means that every time the boss changes so does everyone else. It would be like a sports team getting a new coach and deciding to fire all the players and the coaching staff as well. You need some steady hands at the wheel.
The weakest link is, IMO, the Supreme Court. This was always politicized and where it should be providing a stabilizing limit to prevent everything that has been done from being undone, the political nature of the appointment process means that when you need stability at the wheel, you've got people who are happy instead to spin the wheel madly around in the opposite direction.
That's called reckless driving where I come from. It's hard to imagine the European Court of Human Rights doing a U-turn like that.
The US cannot rely on the courts, it's civil service or it's politicians to reach reasonable solutions to problems by compromising.
That's a barely functioning government.