You can't generate something from nowhere, if you want to set a variable to a random entity you have first to add all the types of entity you want to a list.
after done this you have to set the variable to a random object out of the list and parse it as entity type.

Example:

add "zombie" to {list::*}
add "skeleton" to {list::*}
set {_entity} to a random object out of {list::*}
spawn ({_entity} parsed as entity type) at player's location

 

you're welcome!