When I try to select an island in a uv map it says "Cannot select linked when sync selection is enabled".
I know I have to disable it with this button:

But it is not here:

So where is it?
When I try to select an island in a uv map it says "Cannot select linked when sync selection is enabled".
I know I have to disable it with this button:

But it is not here:

So where is it?
Your second screenshot is of the header in the 3D view, but the setting you are looking for is in the header of the UV/Image editor:

In 3.3.1 I couldn't find the UV Sync Selection or the rest of the toolbar in the UV Editor but realised it was because I didn't have my model selected in Edit Mode. If yours is missing it's probably because you're still in Object Mode.