CTRL+shift+L, in my excel 2007 dosnt work rather it prompts out a dialogue box with a message

While it's true that this would be best answered over at SuperUser, when I Googled this problem myself this StackOverflow question was the first hit. So I'll answer here anyway, now that I've found the solution. :)

It turns out that the Radeon gaming software is the culprit; it takes over a number of hotkeys without so much as a by-your-leave. I found the answer at https://community.amd.com/t5/drivers-software/amd-relive-interfering-with-ctrl-shift-letter-shortcuts-in-other/td-p/276452, though the specific steps listed there are now out of date. At the time I write this, you can find the hotkey settings mentioned in that article by clicking the gear-shaped Settings icon in the Radeon software, then continuing with the process of disabling the hotkeys as the article describes.