Hello! The snippet on the docs had a mistake in it, thank you for pointing it out.
There is a struct type missing before the open curly braces in the Animations field, the proper type definition looks like so:
struct Entity {
Health: u8(0..100),
Position: vector,
Rotation: u8,
Animations: struct {
First: u8?,
Second: u8,
Third: u8
}
}