Kindly do check on the following points. 

1. Kindly make sure you have uploaded your Java function with all the complied class files. 
2. If the class files are present, then kindly make sure you have provided the relative path of the main class properly in the catalyst-config.json file. 

For Example : If your main class is in Sample.java file and the class file for the same is present in /build/class/Sample.class , then the catalyst-config.json will be as follows :

1. {
2.     "execution": {
3.         "main": "build/class/Sample"
4.     },
5.     "deployment": {
6.         "stack": "java8",
7.         "name": "sample",
8.         "type": "advancedio"
9.     }
10. }