I can't place any blocks [closed]

Solution 1:

If nothing is happening when you left click, then your mouse button might not be properly set to attack. Go into Options then Controls and change Attack to your mouse button. Just click twice with your mouse to change it.

It could be you are using /gamemode 2 instead of gamemode 0. Gamemode 0 is survival mode, whereas gamemode 2 switches you to adventure mode. In adventure mode no blocks can be destroyed. (in 1.8 you can destroy blocks if you have the correct tools)

To avoid this you can use the full name /gamemode survival, creative, adventure, spectator. Or an abbreviation of the name (s, c, a, sp).