Convert Docx to PDF using java

We plan to purchase a plan in Convert API and we are doing a POC on converting docx to pdf. I am using a java code available in the link https://www.convertapi.com/blog/file-conversion-library-for-java to convert docx to pdf but i am getting error as "java.util.concurrent.ExecutionException: java.lang.NoClassDefFoundError: Could not initialize class com.convertapi.client.Http". Please help me to fix the issue.