Right. "Negotiation" is both countable and uncountable. The question is, are you talking about continuing an ongoing negotiation, or starting a new one. That is, "further negotiation" (uncountable) or "another negotiation" (countable).
I am talking about continuing an ongoing negotiation,
I am surprised that "further negotiation" is not countable, I thought that it is as there is the word "negotiation" I don´t understand why this difference.
When a noun may be both countable and uncountable, we can say that it may be a thing or it may be stuff.
This is true of a lot of nouns.
In the case we're discussing, you could say, "I think there needs to be a further negotiation." This would mean "another negotiation," or "a different negotiation." (We need to start over.) This would be countable.