Hi Juan,
Type pools : You will come across certain situations in which you need to declare the same set of variables in a number of abap programs . To reduce the pain of this repeatative declarations, you need to declare just one Type-pool and only refer this type pool wherever u need to declare the variables.So in a way, it is global data declaration .
So in your program you are using one of the type-pool.
Thats why you need to declare
Type-pools: SLIS. "at the start of declaration
And your program will run.