Hm weird. Are you able to summon creatures in the simulator and in minecraft?
You might try all these things...
1) Unlocking the vox-l artifact
2) In minecraft, try switching to a different mode. If you're creative mode, try switching to survival (type /gamemode s in minecraft when you're in your world) or if you're in survival mode, switch to creative mode (type /gamemode c in minecraft when you're in your world)
3) you might have gotten opped out somehow
First, make sure your Minecraft username is set.
For example, if your Minecraft username appears on the LearnToMod homepage as 'myusername' but when you look at your Minecraft launcher you see 'Myusername', you'll have mod loading problems or problems running commands.
If it IS set correctly then you can try using the command square on the server admin page to run "op Myusername" Just replace Myusername with your real Minecraft username.