Simple thoughts in a complex life
What to Do When Merge Cells is Disabled in LibreOffice Calc
I found this problem on LibreOffice Calc v3.4.3 Windows. I tried searching the internet, and found the solution, but only after digging for a while.
Turns out that the feature is disabled if you are currently recording changes that happens in the document, as I was doing. So, if you encounter this problem, make sure that the Record Changes feature is not enabled by toggling it from the Edit > Changes > Record menu.
Hope that helps! The original forum thread can be found here.