I believe that scribus is powerful. But I really can't find any way to make professional clean connector arrows between the boxes. Any help will be appreciated
No, Scribus is not made for creating connectors and flowcharts.
But you can use your preferred flowcharts drawing tool, export to svg from there and import the result into scribus as a vector file.
Exporting to PDF or to a PNG and loading the result as an image could also be an option.
In the current development version (1.5.3), you can get the line to snap to rectangles, and it's a bit easier to draw small flowcharts.
But you still won't get the comfort you get from real flowchart programs.