Hi Pat75,
Since you cannot complete a Time Machine backup of your data on your Mac, we recommend that you give the steps in the link below a try.
If you can't back up or restore your Mac using Time Machine
Next, we'd like to show you how to perform the recommended troubleshooting steps via the link above. Please click to open the link, then read through the steps in the section below.
*Make sure that you're using a backup disk supported by Time Machine.
*Unless you're using a network backup disk, make sure that your backup disk is connected directly to your Mac, not through a USB hub or other device.
*If your backup disk is connected directly to your Mac, use Disk Utility to repair the disk.
*If you're using a network backup disk, make sure that the network connection is good and your Mac is connected to the same network as the backup disk. If you completed a previous backup successfully, verify that backup: Press and hold the Option key while choosing Verify Backups from the Time Machine menu in the menu bar.
*Make sure that the firmware of your backup disk is up to date. Check with the manufacturer of the backup disk for details.
*If no other solutions work, start over: Erase your backup disk, then use Time Machine to select it as your backup disk again.
The reason why we are recommending the steps above is because they will show you what to do if you need help to complete a Time Machine backup of your data on your Mac.
If you're still experiencing the same issue, try the following article: Time Machine troubleshooting on Mac
Please let us know the outcome after giving the above steps a try.
Thanks for using the Apple Support Communities.
Cheers!