Does not work

Failed to call hook 'OnActiveItemChanged' on plugin 'SimpleSymmetry v1.1.7' (KeyNotFoundException: The given key 'Square.png' was not present in the dictionary.)
  at System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) [0x0001e] in <8ce0bd04a7a04b4b9395538239d3fdd8>:0
  at Oxide.Plugins.SimpleSymmetry.GetSymmetryImage (Oxide.Plugins.SimpleSymmetry+PlayerBuildOptions buildOptions) [0x00055] in <b41c9a5e00ae4c3f980ad30feaa20996>:0
  at Oxide.Plugins.SimpleSymmetry.CreateSymmetryPanel (Oxide.Game.Rust.Cui.CuiElementContainer& container, System.String anchorMin, System.String anchorMax, System.String offsetMin, System.String offsetMax, System.String backgroundColor, Oxide.Plugins.SimpleSymmetry+PlayerBuildOptions buildOptions, System.String parent, System.String buttonName) [0x0005e] in <b41c9a5e00ae4c3f980ad30feaa20996>:0
  at Oxide.Plugins.SimpleSymmetry.ShowUi (BasePlayer player, System.String buttonName) [0x000b6] in <b41c9a5e00ae4c3f980ad30feaa20996>:0
  at Oxide.Plugins.SimpleSymmetry.OnActiveItemChanged (BasePlayer player, Item oldItem, Item newItem) [0x00095] in <b41c9a5e00ae4c3f980ad30feaa20996>:0
  at Oxide.Plugins.SimpleSymmetry.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00bf8] in <b41c9a5e00ae4c3f980ad30feaa20996>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <d82f1fa409f84967ab685a3dbdd116ae>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000de] in <2a1b2b9b0c9d45ea87d32bff811299cc>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <2a1b2b9b0c9d45ea87d32bff811299cc>:0

can only use chat commands to get this to work

Add the  assets into your data folder from the zip. See the readme for details

Changed Status from Pending to Not a Bug

With the ASSETS folder loaded, I am also getting this error when I input "/sym ui"...
UPDATE: The plugin isn't even working. Based on the install guide you have provided - The plugin is not functional, none of the "/sym (type)" commands are actually doing anything when trying to place. 

Are you using this on a creative server? IE without materials in your inventory? In this version you need to have the materials in your inventory. I will be making an update for creative servers that dont want to have the materials in their inventory.

If you still see this error, unload the plugin and then delete the /Data/SimpleSymmetry.json file and then load the plugin. This should reload the images from the assets folder.