So you want, or been advised, to use Additional CSS in the Customizer and you can’t find it anymore?
As of WordPress 6.0 hide the Customizer when no theme or plugin need it. WordPress deemed that Block Themes by default do not need the customizer, so if you have a block theme like TwentyTwentyTwo you may find the customizer gone!
There is a simple solution, basically tell WordPress you need it by a little bit of code. But of course a little bitof code means getting technical, do you create a child theme etc. The simple solution is a plugin that has that tiny code snippet.
Just install and activate to get the customizer back.