The Netherlands are located in the north European plains. This area is formed by a crystalline basement generally about 4km deep, covered by more or less horizontal Mesozoic sediments. The sediments are typically marls, limestones, dolomites, and at the bottom salt and gypsum. The surface is mostly flat, covered by quarternary sediments, influenced by the glaciers of the Ice Ages. Most of the sediments are deltaic, coastal and eolian sediments. The deltaic sediments are deposited by the four main rivers: Rhine, Meuse, Schelde and IJssel. This area was not tectonically altered and so there are no hillcountries. At the southern borders are some foothills of the Ardennes.
It is easy to understand, why this geography does not permit karst and caves. The whole land drains on the surface, it is too low to allow subterranean drainage. Even if there are caves, probably formed during the Ice Age when sea level was 100m deeper and the channel to Great Britain was dry land, this caves would now be full of water. However, none of these caves are known and so this possibility is theoretical.
Probably you have heard the pop song about the dutch mountains, which really do exist, at the southern border to Germany. But the name is a sheer exaggeration. This mountains are a nice undulating, but low, hill country. But here is really the only part of the Netherlands were one can find caves. Beneath a few natural caves there are thousands of artificial caves, which are locally called caves too. They are not natural caves, but marl and limestone quarries. The people mined the limestone for various purposes, typically burning it for lime and cement or using it to build houses and churches.
The Netherlands are said to be a country with exactly one natural cave. This ist not true, there are only natural cave of the country is not open to the public.
Nevertheless, the Netherlands have a show cave. As a remains of its history as a naval power, they have still some colonies. On of them are the Netherland Antilles, in the Carribbean Sea. And those islands have numerous caves, one of them open to the public.