according to the screenshot you haven’t selected a source folder
The mandatory argument is the directory where the input files are located. The given path can either be absolute or relative. If a relative path is given, the processor expects to be called from the path where the input files are located. Otherwise it will show an error message, that the directory cannot be located. If an absolute path is given, it must point to the directory where the input product is located.
Source: 1. Software Installation and User’s Manual (SUM) — Sen2Three 1.1.0 documentation