Please use the Insert Code Snippet button when adding code examples to your questions / comments, it makes them easier to find and read.
The first two lines of your example are assigning Number values to the $fullhealth and $health variables
(set: $fullhealth to 10)
(set: $health to $fullhealth)
The 3rd line is assigning a (random) String value to the $damage variable.
(set: $damage to (either: "2", "3", "4"))
The 4th line is trying to delete the String value from the Number value, which results in an error becayse the data-type of the two values is different.
(set: $health to it - $damage)
The solution is to change the String values within the 3rd line to be Numbers instead.
(set: $damage to (either: 2, 3, 4))