Running FFmpeg in Automator/AppleScript
An important consideration when programming is to come up with a plan for debugging your code.  Here is how I would have coded the code for debugging.

It's difficult to debug an on open handler.  I add an on run handler where I can dummie up the input to a common routine that is called by on run and on open. fyi: I corrected how the input was processed from on open. 

(* 
   AskDifferent question:
     https://apple.stackexchange.com/questions/366992/running-ffmpeg-in-automator-applescript
 Use log statements.

 It is easier to diagnose problems with debug information. I suggest adding log statements to your script to see what is going on.  Here is an example.

For testing, run in the Script Editor.
    1) Click on the Event Log tab to see the output from the log statement
    2) Click on Run

 *)
on run
    --  debug lines
    set desktopPath to (path to desktop) as string

    -- here is a log statment.
    log "desktopPath = " &amp; desktopPath

    -- Be sure to select a file on your DESKTOP.
    set see to alias (desktopPath &amp; "auto-show-2015.mp4")

    -- Simulate dropped items list.
    set dropped_items to {see}

    common(dropped_items)

end run


-- Gets invoked here when something is dropped on this AppleScript icon

on open dropped_items

    common(dropped_items)

end open



on common(dd)

    repeat with d in dd
        set d to d's contents
        log d
        tell application "Finder"
            --set seq1 to (d's file 1 as alias)  -- dropped items come in as an array of aliased items. 
            set seq1 to d
            set dparent to d's container as alias
            set mov to "" &amp; dparent &amp; (dparent's name) &amp; ".mov"
            log "mov is " &amp; mov
        end tell

        set toUnix to "d=" &amp; d's POSIX path's quoted form &amp; " /opt/local/bin/ffmpeg -r 25 -f image2 -i \"" &amp; seq1 &amp; "\" '*.JPG' -codec:v prores_ks -profile:v 0 \"" &amp; dparent &amp; ".mov\" &amp;&amp; exit
    "
        log "toUnix is " &amp; toUnix
        set fromUnix to do shell script toUnix
        log "fromUnix is " &amp; fromUnix
    end repeat
end common