I was also struggling with this! I was convinced that I had done something to my Xcode 15 install which caused this option to disappear, but it turns out that this is just a big change that was made in Xcode 15:
The Show Code Actions command has been replaced with Show Quick Actions to quickly access any menu command. By default, Command-clicking a token in the editor now performs Jump to Definition. This can be changed in the Navigation preferences. Control-clicking a token brings up the standard contextual menu that now contains all the commands that were available in the Code Actions. (86179596)
Xcode 15 Release Notes: New Features
Apple wants you to either use the new ⌘+Shift+A keyboard shortcut to access these options, or to just right click and access them there.
Topic:
Developer Tools & Services
SubTopic:
Xcode
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