How to develop Custom Keywords Plugins

I made junit4ks-all.jar.

I created a brand new project using Katalon Studio 6.0.6. I. I created Plugins directory in the root directory of the project. I copied the junit4ks-all.jar into the Plugins diretory. I reopened the project, and then I got the “Unable to open project” error dialog as follows:
UnableToOpenProject

I attached the junit4ks-all.jar (renamed to be *.zip because this forum site rejected *.jar file to be attached)
junit4ks-all.jar.zip (37 KB)

Please help fixing this error.