Make sure your add-on does not have an add-on folder inside it.
Example Correct Addon Folder :
Client_MyGUI (folder / zip)
|
|-mygui.gui
|-client.cs
|-description.txt
Example Incorrect Addon Folder :
Client_MyGUI (folder / zip)
|
|-mygui.gui
|-client.cs
|-description.txt
|-Client_MyGui (folder / zip)
|
|-mygui.gui
|-client.cs
|-description.txt
Do not have an add-on inside an add-on.