If you can't back up or restore your Mac using Time Machine
If Time Machine states that it can't use your backup disk to back up or restore your files, try these solutions.
Make sure your backup disk is connected directly to your Mac, unless you’re using a network backup disk. Don’t connect it through a USB hub or other device.
If you’re using a network backup disk:
Make sure the network connection is good and your Mac is connected to the same network as the backup disk.
Check for VPN software or other third-party security software that could be affecting the connection to your backup disk.
If you’ve completed a previous network 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 the firmware of your backup disk is up to date. Check with the manufacturer of the backup disk for details.
Find out what to do if Time Machine is working but takes a long time to back up.