i think its because you're at the vex surface shop level , you should add it at the basic liquid shader…(just go one level up pressing the “u” key, and then try to add the vex fractal dent.)
hmm am pretty sure it is a houdini9 tuorial, i had spotted that the fractaldent could be added to the shop but didnt have the other nodes in there diagram.
figured it out thou, missing from the tutorial is the need to turn the basic liquid into a material with Shift-C, then you can dive in (press i) and add the node and continue the tutorial as normal.