Hi @cakhaing
The build process seems to be using Solution Packager. So, folder structure needs to be in a structure acceptable for Solution Packager.
So, I had to do the below to get this working.
1. Add new node in Solution.xml for the plugin assembly
2. Create the folder structure that matches what you specified in the Solution.xml

3. Below is the xml I used.
<?xml version="1.0" encoding="utf-8"?>
<PluginAssembly FullName="pacplugin, Version=1.0.0.0, Culture=neutral, PublicKeyToken=086a4ebfa030abc1" PluginAssemblyId="f0d86b48-0377-4697-81a0-ee054aa16bff" CustomizationLevel="1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<IsolationMode>2</IsolationMode>
<SourceType>0</SourceType>
<IntroducedVersion>1.0</IntroducedVersion>
<FileName>/PluginAssemblies/pacplugin-f0d86b48-0377-4697-81a0-ee054aa16bff/pacplugin.dll</FileName>
<PluginTypes>
<PluginType AssemblyQualifiedName="pacplugin, Version=1.0.0.0, Culture=neutral, PublicKeyToken=086a4ebfa030abc1" PluginTypeId="abfc98bc-f290-4f52-949b-eb37af1c992a" Name="pacplugin.Plugin1"></PluginType>
</PluginTypes>
</PluginAssembly>
Once I did these steps I was able to build the project.

This process appears to be very clunky at the moment. Maybe it is not meant to be used for general consumption yet.