About Undo Block
Download the UndoBlock.cs. undoblock.use allows you to use this plugin.
Put it in the oxide/plugins folder.
And everything should be okay.
If your server doesn't create config file:
If you press 'CTRL + R', the last block you added will be removed.
"Refund Block": false
If you want the materials to return to the player, you can set the setting to true in the config file. Default setting is false.