Time machine "can't complete backup because some files were unavailable"?

Hello Sarah_in_SL,

Thanks for using Apple Support Communities! We understand you are having some issues with Time Machine backing up your Mac. We are happy to help provide some guidance.

If you haven't configured Time Machine on your Mac yet, take a look at the steps to set it up to start creating backups here:

Back up your Mac with Time Machine

If Time Machine runs in to any issues creating backup files, take a look at the steps outlined in the following article that goes over some things to check:

If you can't back up or restore your Mac using Time Machine

Time Machine troubleshooting on Mac

If the issues persist, we'd like to ask some additional questions to better understand what is happening.

• What kind of disk are you using to store your Time Machine backups? Are you using the hard drive inside of the Mac, or are you using an external drive?
• Have you made Time Machine backups successfully in the past on your Mac, or is this the first time you are trying?

Keep us updated on how it goes or if you have any questions.

Best Regards.