Can I install the cabinet inside a cupboard or other enclosed space?

Yes, but think about what happens to the heat after it leaves the cabinet.

A roof fan can extract warm air from the rack into the surrounding cupboard. If the cupboard itself has very little ventilation, you haven’t actually got rid of the heat — you’ve merely promoted it from one box into a slightly larger box.

As that surrounding space warms up, the air being drawn back into the cabinet becomes warmer too.

For a low-power installation this may be insignificant. With PoE switches, UPS units or several active devices, ventilation of the surrounding cupboard can become just as important as ventilation of the cabinet itself.

A fan moves heat; it doesn’t make heat cease to exist.