I mean you can see on the image that I already have "12 claims" on them (and yes I'm sure).You're sure you've got claims on them?
The image you've posted suggests you don't have any.
Check the claims menu at left and make sure you've got claims on them.
It might be worth specifically making a claim just to check.
If you actually opened the image you can see that I have 12 claims on them. The problem appeared after I demanded them to be my vassals so I can get the tributary/vassalize casus beli just for fun.I'll start with the most obvious potential issue and we can go from there.
Have you made any claims on this empire's territory? You can only use the Conquer Casus Belli if you have claims on the territory of the empire you are attempting to use the Casus Belli on.
Left side of the UI is not modded you simply press the small button on the bottom so the icons will not expand when hovering over them (also I'm using 2560x1440 resolution so I guess this helps with the things to look smaller as well). Right side of the UI I'm using Tiny Outliner v2 mod ( https://steamcommunity.com/sharedfiles/filedetails/?id=1610578060 ), first campaign using it and I already love it ! Imo it should be added by the devs as the default UI.Never happened to me before, and given the screenshot you've provided I'd say it's a bug. Also, unrelated question, for the UI bar on the left, are you using a mod or is it an option I've been too blind to discover ? Because I wish I had such a compact overview of my empire.
Left side of the UI is not modded you simply press the small button on the bottom so the icons will not expand when hovering over them (also I'm using 2560x1440 resolution so I guess this helps with the things to look smaller as well). Right side of the UI I'm using Tiny Outliner v2 mod ( https://steamcommunity.com/sharedfiles/filedetails/?id=1610578060 ), first campaign using it and I already love it ! Imo it should be added by the devs as the default UI.