So I just bought all 9 Pieces of the Treasure Map and put them together. I can't visit the Omen Islands, what did i wrong?

You did the wrong map. You need to complete the Uncharted Land Map to get to the Omen Islands.

So i just paid 1.6 mil. for nothing :/ Thank anyway :)

On the bright side you are one step closer to getting access to the Blue Building.

Thats true, thanks for the information :)