Time Machine troubleshooting on Mac
Time Machine must create a new backup
The backup disk is unavailable or can’t be found or “mounted”
An incomplete, interrupted or failed backup occurred
Time Machine is slow or seems stuck
Make sure your backup disk is plugged in, turned on, and securely connected to your Mac. If your backup disk is on a network, make sure both the disk and your Mac are connected to the network. If the network is experiencing problems, your backup disk may not be available.
Download and install all recent updates to macOS. If you have a Time Capsule, update it with the latest software and firmware. See Keep your Mac up to date and the Apple Support article Update the firmware on your AirPort base station.