Hello and thank you for your question. I look forward to assisting you today. This may be caused by some malware locking a file needed for your backup. Go to this LINK and install MalWareBytes. Run a scan and delete everything that is found then try your backup again. If you have any antivirus software running, disable it first as the two programs may conflict. Let me know if you need further assistance.
Let's try this first. Go to this LINK and install the Microsoft Safety Scanner. Do a full scan of your system. It may take some time so be patient. Once the scan is complete remove anything it detects. After that, do a Windows update. After the update is complete, restart your computer and try your backup again.
Outstanding! Glad it worked for you. Have a nice day!