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I have periodically used my time machine for years now. The last time I backed up was November 2017 and I started to try and back it up last night and it took ages to "prepare" and started to back up, but then kept failing. When I click the red (i) symbol on my TimeMachine it just says

Time Machine couldn't complete the back up.

There's no further answer as to why. I ran it a few times and noticed that it now says that the last successful backup was June 2015. So I don't know what happened to the other 2 years worth of backups. It was still saying last back up was November 2017 on previous failures yesterday.

How do I resolve this?

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It sounds like you may need to dive into the logs to get some more info.

MacOS uses unified logging so the logs aren't stored in a specific file. In Terminal, this command should print all TimeMachine backup failure messages:

log show --predicate 'subsystem == "com.apple.TimeMachine"' --info | grep 'Backup failed'

However this cuts out quite a bit of information, so you may need to use the broader search:

log show --predicate 'subsystem == "com.apple.TimeMachine"' --info | grep 'Error'

Running this on my machine showed TimeMachine to be quite noisy so it may take some time to find something useful in the output especially if you have been running TimeMachine for a while

Post back here with any errors you find.

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