You (I) don't specify the flat file name in Planner; Planner uses the plan file name and adds "-flats" to it, then creates the file.

Your plan has this line:
#duskflats ; Acquire flat fields at dusk

My plans created by Planner used to have this line:
#duskflats xxx-flats.txt ; Acquire flat fields at dusk

Now they have this line (note the full path):
#duskflats C:\Users\mikey\Documents\ACP Astronomy\Plans\xxx-flats.txt ; Acquire flat fields at dusk

I don't know why your Planner didn't add a flats filename to your plan, or create that file.

--- Mike