There is some new UI in Xcode for upgrading OS versions. I clicked the new Get button at the top of the screen. It went and got the new iOS 17.4. Great, but it is just sitting there. After much floundering around, I eventually found the UI that has the Install button. Clicked it. Nothing happens. I have restarted Xcode several times, went looking for an Xcode update, no joy. My development is now dead for two days. Really not happy.
Show us a screenshot of the Platforms tab in your Xcode Settings.
For me, I removed every simulator runtime except the shortest named ones, i.e. I removed "iOS 17.4 beta 2 Simulator" and left "iOS 17.4 Simulator" alone. Then I clicked on the "Get" button by the remaining items, and it worked.
Here it is.
Right, so delete the one called "iOS 17.4 Simulator" and then click "Get" on the "iOS 17.4" item above. The same thing applies for "visionOS 1.1 Simulator".