Time Machine Issue

Have been successfully running Time Machine twice a week backing up my iMac files to a Seagate External Hard Drive attached via USB port. Got a message today that scheduled backup could not complete and that files could not be backed up to hard drive possibly due to anti-virus software. I have had Norton also running for last 2 years with no problem. I completely turned off Norton, restarted and requested Time Machine to do back-up but got sane error message. Did research and found out about the issue of having an “.inprogress” file in the back-up folder used by Time Machine. I do have such a file created today I assume from a failed backup attempt. I tried delete this file using finder but when I attempt to move to trash, it requests my admin password and when I enter it, it gives me an error message saying I can’t move to trash because it may be locked or blocked from access and to check using Get Info. I checked and the fije is not locked and I don’t believe access is blocked. Can someone please tell me a full proof way to delete this inprogress file from the Time Machine backup folder? Or more importantly, how can I get Time Machine doing scheduled backups again to my external hard drive? Thanks.