Thanks for reaching out. If you can clarify a few things for me, we can get to the bottom of this.
Could you please share the version of Lightroom you are working on by Help > About Lightroom Classic and the operating system you are working on? Please specify if you are using Lightroom Classic or the computer in a regional language apart from English? Do you have a regional keyboard?
Please use the OnScreen keyboard on Windows or Accessibility Keyboard on macOS to check if the input for those specific brackets is working. You can also test by pressing Shift key + [ ] braces to check if the feathering on the brush changes or press (Win) CTRL + [ ] braces (macOS) CMD + [ ] braces to rotate the image.