After creating a radial mask by pressing R, Lightroom tells me to press Shift+R to add another radial mask to the mask I just created. But whenever I do so simply nothing happens. Same issue with adding Brush or Linear masks to an existing mask. I really don't want to click on the plus item and then on Add to Mask every single time. The shortcut could save a lot of time!
Thank you for the quick answer! Unfortunately this is the exact way I tried doing it... Maybe I need to update my Mac OS, I am still on 11.5.2, because everytime I press Shift+R my Macbook does the Mac Plopping Error Sound. Or is there any way to reset all settings inside Lightroom?
The symptoms you describe, the beep, sound similar to a hidden dialog having focus. That would be a first place to look. Restart your Mac - open only Lightroom and make sure it is not covering the entire window - attempt the shortcuts and see if there is a dialog lurking somewhere.
Other than that, I would look for applications that may have hijacked the shortcut for their own uses. Copy/Paste utilities, Screen Capture Utilities, and others can sometimes consume active KBSCs.