How do I get claims to wage war?

[Shampoo]

I am playing as Japan but I don’t have a claim to the small island area that is above Japan, when I try to put an interest in that area it says that I already have an interest in Japan but I don’t own that area…

[6ap6apblckaAa]

What exactly do you want? Claim is only needed for less infamy when waging war. And it also blocks colonization.
If you want to start a war, just lower your relationship to neutral and start a diplo play, say, with a conquer state wargoal.

[6ap6apblckaAa]

If you want to colonize claimed territory, first start a war with a revoke claim wargoal against the holder (probably Russia in your case).

[Loffas]

They are a thing in this game. Only the computer can make them, it is a oversight and the devs forgot about adding it as an diplomacy option for players. Like everything else that remotly touches warfare in civ 3 it is broken

[terchong004]

Sakhalin? You can colonise it after fully colonising Hokkaido and absorbing the Ainu nation. Claims are not necessary in this game. You can conquer any state without claims.

[Alex]

But you can't colonize the Kuril islands (those between Kamchatka and Hokkaido) due to a bug. Use this mod as a temporary fix:
https://steamcommunity.com/sharedfiles/filedetails/?id=3099599161