Backup trouble: snapshot of disk not created.

That message means Time Machine was unable to create the APFS snapshot it needs before it can start the backup. 

Apple notes that this can happen when the Mac’s internal disk is nearly full, so I would  first check how much free space is available on Macintosh HD and free up some space if necessary. 

Time Machine stores its local snapshots on the Mac’s internal APFS disk. (support.apple.com)

The Toshiba drive being formatted as APFS (Encrypted) is a supported format for Time Machine, so the formatting itself should not explain the error. The message also does not, by itself, indicate that the Toshiba drive is defective. (support.apple.com)

If you have plenty of free space, Apple recommends connecting the backup drive directly to the Mac, checking it with Disk Utility, and making sure its firmware is up to date. 

If you have third-party VPN or security software installed, Apple also recommends checking whether it could be interfering with Time Machine. (support.apple.com)

As for legacy software, I would not assume that it is the cause. This error alone does not establish that, and Apple does not identify legacy software as the cause of this particular snapshot problem.

I would hold off on erasing the new Toshiba drive for now. 

Apple lists erasing and setting up the backup disk again as a later step if the other troubleshooting does not resolve the problem. (support.apple.com)

For more detail, see Apple’s guidance on Time Machine backup failures and troubleshooting Time Machine backups.