Why doesn't my caramel sauce thicken?

Sugar "boils" at a temperature much higher than water. The water is there just to get things started, so you don't burn the non-melted sugar directly in contact with the pan at. Within a few minute the water will have evaporated, and all you have left is liquid sugar

Use a candy thermometer and slowly raise the temperature until you get the sugar colour and flavour you want

When it cools it will thicken, or become solid, depending on how high you heated it. It is not just a time thing, it is the maximum temperature your reached

Caramelisation with happen around 170°C (340°F) depending on what type of sugar you use. Caramel sauce usually requires the addition of a fat like cream

I am curious, what kind of vessel are you using for this procedure?

I notice that you are working with very small measures here. 24 g of liquid will coat a normal sized pan in a film-thin layer. I cannot imagine this behaving well, or being controllable at all.

The original amount would be doable in a 12 cm "buttermelter" saucepan. The amount you chose will probably not function in anything larger than a muffin cup, ca. 6cm. And I don't know if you can even get individual muffin cup nowadays, they get sold welded into tins.

Basically, when you make candy, try having at least 1 cm of depth of the liquid when you pour it into the heating vessel. Anything else is practically impossible to work with.

If your problem was only the thinness, I would expect it to scorch quickly, maybe too quickly for you to see it going through a boil. But if it had scorched, I expect that you would have mentioned it. Your question is worded as if nothing seems to happen in your pan. If this is the case, then your temperature is too low. "Medium high" is not a meaningful direction unless you are so good at making the food in question that you can predict its behavior at different temperatures, and can adjust the temperature so it behaves such that it is at a bit above the middle of the heat range it can tolerate. This does not correspond to a specific setting of a stove, but varies between stove brands, batch sizes, pan types, etc. Just forget it and use a candy thermometer, as TFD suggested.