Time Machine backup is too large (but shouldn't be)
I know what is the problem... I only have one backup on time machine and it won't delete it because it's the only one. So I can either clean up some space on the drive (I have other things there) or just delete the backup and let it run again.

How to delete a backup - from here:

The easiest way is to:


Enter time machine (on the Mac whose backup you want to delete)
Go to the point in time you want to delete
Select the icon that looks like a cog in the finder and choose 'Delete Backup'
This ensures the backup catalog remains accurate and the integrity of your data stays intact.