“List of backup timestamps contains duplicates” due to DST change describes the DST failure, why deleting versions is seemingly impossible without source edits, and a possible workaround of database editing which hasn’t yet been proven (maybe you’ll be the first?). Regarding your current message though, you might just need to go over to job Database option to check “Local database path” so the command knows where to go using your addition of –dbpath. I’m not sure how Duplicati generated what you saw, but just give it the path…
Two easy methods that also pick up other parameters you might want (but didn’t give) are the Commandline option of the job, and the job Export --> As Command-line. Either way, you get to turn a backup into a delete then possibly run into the problem described in the link above (assuming yours has the symptom those had).