Do you have any keyboard add-on program installed?
Have you made any changes to the language keyboard setting in the OS you are using?
What is the OS you are using?
In which language are you running LR? French and Swedish have known problems with the "x" shortcut:
If you're in one of those languages, see the appropriate topic for a workaround and add your me-too vote and opinion to that topic. If you're in a different language, please report it here.
I have the same problem. I have no keyboard add-on programme. And, yes, I use a French keyboard
J'ai le meme problème que toi et j'ai contacté le support adobe par chat ce matin meme. Il semble que cette modification ne soit pas un bug... Ce qui est très embêtant au quotidien ! As-tu eu une autre réponse/solution ?
I think Adobe support is giving out incorrect answers. Adobe marked the bug report in the official Adobe feedback forum as "In Progress", meaning they've acknowledge the bug and are working on a solution. Meanwhile, see this post in that bug report for how to work around the problem while waiting for a fix: Lightroom CC 2015.8: Shortcut X to reject an image in Library not available (French) | Photoshop Family Customer Communi…
Please update Lightroom to 6.9/2015.9 version which includes the fix for Lightroom CC 2015.8: Shortcut X to reject an image in Library not available (French) | Photoshop Family Customer Communi…