The error message "You are using docker; download client Transmission places downloads in /downloads/complete but this directory does not appear to exist inside the container. Review your remote path mappings and container volume settings."
I had same issue, and that was a mistake I had made when sym-linking the transmission download directory to my home/user/ directory, I changed the ownership of the sym-linked file which by consequence also changed the ownership of the transmission 'download' directory...
Remote Process Groups provide a mechanism for sending data to or retrieving data from a remote instanceof NiFi. When a Remote Process Group (RPG) is added to the canvas, it is added with the Transmission Disabled,as indicated by the icon () in the top-left corner. When Transmission is Disabled, it can be enabled by right-clicking on theRPG and clicking the "Enable transmission" menu item. This will cause all ports for which there is a Connectionto begin transmitting data. This will cause the status indicator to then change to the Transmission Enabled icon ().
There are times when the DFM may want to either enable or disable transmission for only a specific port within the Remote Process Group. This can be accomplished by right-clicking on the Remote Process Group and choosing the "Manage remote ports" menu item. This provides a configuration dialog from which ports can be configured: