Hello sperry1975, welcome to Seven Forums!
Since the back-ups seem to be completing successfully, perhaps you would have better success creating a second, separate task to shut-down the PC after the backup task has completely finished.
Have a look at this tutorial at the link below and be sure to post back with any further questions you may have and to keep us informed. Task Scheduler : Create Shut Down PC Automatically Task
You may have to add " /s /f
" (force) to the arguments dialog.