Time Machine troubleshooting on Mac
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.
If you upgraded to macOS Big Sur on a Mac that uses a Time Capsule or other network storage device as the backup destination, your existing backups are also upgraded and can be used only on macOS Big Sur. New backups that are created can be used only on macOS Big Sur.