Time Machine could not create a local snapshot to back up from.

Start with this...

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

Oh, Holy Cow, that is it!

You need at least as much free space as you have RAM plus how much RAM all the Apps you run use up!

And that will be cutting it close!!!

You're in danger of loosing everything!

Have you emptied the trash lately?

Terminal code to clean DocumentRevisionsfolder…

https://apple.stackexchange.com/questions/313102/what-will-occur-if-the-documentrevisions-v100-folder-is-deleted

macos - What will occur if the .DocumentRevisions-V100 folder is deleted? - Ask Different (stackexchange.com)

https://discussions.apple.com/thread/253231342?answerId=256043117022#256043117022

System Memory OS 10.12.6 Sierra - Apple Community

Look for iOS backups…

/Users/YourUserName/Library/Application Support/MobileSync/Backup

OmniDiskSweeper shows you the files on your drive, largest to smallest, and lets you quickly Trash or open them.

https://www.omnigroup.com/more/

Purging local backups

Please note that although this doesn't affect your remote backup from Time Machine, this will get rid of the redundancy (at least until the next Time Machine backup) that a local backup disk will provide. If you need such redundancy or are worried about the recovery of your data then you would be best served to let macOS determine when to purge these files.

Start Terminal from spotlight.

At the terminal type tmutil listlocalsnapshotdates. 

Hit enter.

Here, you'll now see a list of all of the locally stored Time Machine backup snapshots stored on your disk.

Next you can remove the snapshots based on their date. I prefer to delete them one at at time. Once my "System" disk usage is at an acceptable level, I stop deleting but you can delete all of them if you want to reclaim all of the disk space.

Back at the terminal, type tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS , where will be one of the dates from your backup. This will be in the form of xxx-yy-zz-abcdef. Try to start with the oldest snapshot.

Hit enter.

Repeat for as many snapshot dates as required

http://www.thagomizer.com/blog/2018/03/27/cleaning-up-time-machine-local-snapshots.html

tmutil deletelocalsnapshots /  # deletes all the snapshots

It worked for me in safe mode. So do I need to re-do in non-safe mode? and does it mean something if it doesn't work in non-safe mode? I don't have a lot of available space. I was hoping to move stuff off after doing a backup.

Here you go my friend...

Move your Photos library to save space on your Mac - Apple Support

Didn't work in safe mode. Do you have another suggestion?

How much free space is on your Hard Drive?

Space is critical, Safe Mode gives just a bit ore space, but do a Get Info on your drive, how much is little?

3.44 GB available out of 121.12 GB if that helps - looked at storage

I empty trash multiple times a day - can I just "move" stuff from my "finder" to the new external drive? I was hoping I could move documents and photos.