Time Machine cannot back up because of insufficient space on the source volume.
My MacBook Pro won't enable Time Machine back up because of insufficient space on the source volume, according to the error message.
MacBook Pro 15″, macOS 10.13
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1) It sounds like you need a larger external drive for Time Machine.
see > If the Time Machine backup disk for your Mac is full - Apple Support
If the Time Machine backup disk for your Mac is full
If you do run out of space, it’s best to connect a new backup disk. After you connect the new disk, open Time Machine settings, then select it as your Time Machine backup disk.
2) The external drive for Time Machine should be twice the size of the Mac's internal drive.
see > Back up your Mac with Time Machine - Apple Support
Connect a storage device to use for backups
- Use this storage device only as your Time Machine backup disk, not for storage of other files.
- Ideally, your backup disk should have at least twice the storage capacity of every disk or volume you're backing up. If your backup disk doesn't have enough storage space to contain a complete backup, Time Machine will let you know.