Time Machine couldn't complete the backup to "File Storage" ... because a snapshot of the disk could not be created

Most often this happens when there is not enough space on your internal drive (the one being backed up, not the Time Machine destination drive), or if there is a file system problem or misconfiguration on the internal drive.

Open Disk Utility. You should see something like this. Make sure under View (upper left of screen) you select the drop down menu option to Show All Devices.

Note the example below shows a lot of space available. How much space is available for you? If you have plenty of space, then try booting into Recovery Mode (COMMAND-R while booting) and select Disk Utility and run Disk First Aid on all volumes and devices (must select "Show All Devices").