Hello! And welcome to the Fanfiction & Writing forum.
This is only a prologue, so there's only so much I can say about this story. But I can see a few things that need to be said right away.
It doesn't look like a bad start. It does make me want to read on to see what happens next - which is surely the measure against which all prologues ought to be measured. So that's good: it's what you want from this.
But - and there is always a but - there are several problems with it that need to be addressed.
Most urgently: your protagonist asks us to believe that every single Pokémon that might come into contact with humans has been sealed away. Let's say there aren't as many wild Pokémon as I think there are - even so, how would one group have been able to do that? Over the entire world? Surely people at the time would have resisted - and this is a scheme that would have required the cooperation of the entire global population to pull off. I'm not sure how it would have worked at all; Team United would need incredible resources to make their plan reality.
Perhaps you've thought of a way for all this to work - I don't know for sure. It
is only a prologue, after all; there might be more explanation to come. If so, I look forward to it; I want to be surprised and amazed by the ingenuity of United's plan. That would be fantastic. If there isn't any more explanation, the idea might need some thinking about
: as it stands, it's not terribly convincing.
A couple more specific things:
These captors always have with them a PokeLock and a strange steel ballpen when they're on the fieldwork, and they wave their ballpens strangely every after their work has been done.
This seems oddly specific. You might want to leave that part out until after the Prologue - it's something the reader should be shown as it becomes relevant, not told all at once at the start. The idea of a prologue is to give us just enough to whet our appetite - not to tell us so much we don't need to read the rest of the story.
There are a few oddly used words, too. 'Let's not focus about the name' should really be 'Let's not focus
on the name'; the word 'about' just can't be used that way. In the paragraph above, you write 'Did they already forgot', which should be either 'Did they forget' or 'Have they already forgotten'. These aren't massive things, just little slips that are easily corrected and don't detract much from the overall message.
But for all that, it seems like the story's going to be epic in scale: restoring the balance of nature; a single kid taking on a Team that, in order to have achieved even what it accomplished long ago, must rival the armies of Genghis Khan in size; an entire world waiting to be freed - it certainly seems like it's going to be a large-scale, exciting read. I want to know more about Sen, and how he knows what he does; I want to know more about the Team, and how they did what they did; I want to know more about the people of this world, and why they don't seem to care that the Pokémon are disappearing. And making me want to know what happens next is what a prologue's all about.
So! I look forward to seeing where you take this idea. Good luck, and I hope the story goes well for you!