Invalid bug, moving this thread to Questions for further community assist.
Furthermore, make sure all these things when you are using World Edit:
1- You can ONLY use World Edit in the Admin's World and/or in plots/vip plots you have been TRUSTED (/p add won't work, it MUST be /p trust)
2- Since you are now just learning to use this amazing tool, make sure you are in cuboid select mode (Do //sel cuboid to confirm), and then make sure to select a first and second position with your wand. If you have done this successfully, it will tell you how many blocks you have selected in your boundaries: "Second position set to (COORDINATES) (NUMBER OF BLOCKS)". If you are World Editing in a plot, ALWAYS make sure that your selection is confined WITHIN the plot and not outside it. Elsewhere, it won't work!
3- When you are having problems with the //Set command, try firstly writing the block ID rather than the block and durability name, since most
MATERIAL NAMES differ from the names you see in the Creative Inventory so this may bring you confusion. For example, use //set 159:14 for Red Stained Clay.
4- For further help regarding World Edit, I highly recommend you to watch a video tutorial since WE is a plugin with loads of things and content which can be tricky to learn all at once. This is a good tutorial you may want to watch for more information regarding this amazing tool:
Hope this helped you!