Time Machine troubleshooting on Mac
The backup disk is unavailable or can’t be found or “mounted”
See:
If Time Machine on your Mac can’t find your backup disk
If you need a password to unlock your backup disk on Mac
Apple Support article: Time Machine - Backup volume could not be mounted
An incomplete, interrupted or failed backup occurred
See:
If a Time Machine backup on your Mac is interrupted
Time Machine is slow or seems stuck
See:
If the Time Machine backup disk for your Mac is full
General troubleshooting
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.