DiskManagement error -69546 converting to APFS under High Sierra

The original 1 TB caviar disk on my MacPro5,1 was getting full so I decided to replace it with a new 4 TB SSD drive. I am running macOS High Sierra Version 10.13.6.

1. Remove original HDD and replace it with a brand new unused Samsung SSD 860 PRO 4TB
2. Boot in recovery mode and format SSD as APFS encrypted
3. Still in recovery mode, restore disk from Time Machine backup
4. To my surprise after the long restore, the disk is formatted Journaled HFS+, as it was in the HDD
5. Still in recovery mode, run Disk Utility and run Repair Permissions, then Repair Disk
6. Restart.
7. Boots fine, everything seems to work, but file system is Journaled HFS+
8. Boot in target mode and from a MacBook running High Sierra 10.13.6, run DiskWarriror 5.2, and rebuild directory
9. Still in target mode, from a MacBook run Disk Utility and run Repair Permissions, then Repair Disk
10. The Convert Disk to APFS command is still grayed out
11. Try trick to unmount disk and convert, but this produces the error "The operation couldn’t be completed. (com.apple.DiskManagement error -69546.) Operation failed…"
12. Create a Mojave installer USB stick and boot from it
13. run Disk Utility, but Convert Disk to APFS command is still grayed out
14. Strange fact: System Information states TRIM Support: No

What is error -69546 and what did I do wrong?

Samsung SSD 860 PRO 4TB:

  Capacity: 4.1 TB (4,096,805,658,624 bytes)

  Model: Samsung SSD 860 PRO 4TB                 

  Revision: RVM01B6Q

  Serial Number: ********     

  Native Command Queuing: Yes

  Queue Depth: 32

  Removable Media: No

  Detachable Drive: No

  BSD Name: disk0

  Medium Type: Solid State

  TRIM Support: No

  Bay Name: Bay 1

  Partition Map Type: GPT (GUID Partition Table)

  S.M.A.R.T. status: Verified

  Volumes:

EFI:

  Capacity: 209.7 MB (209,715,200 bytes)

  File System: MS-DOS FAT32

  BSD Name: disk0s1

  Content: EFI

  Volume UUID: 0E239BC6-F960-3107-89CF-1C97F78BB46B

Tofari:

  Capacity: 4.1 TB (4,095,945,900,032 bytes)

  Available: 3.43 TB (3,428,243,939,328 bytes)

  Writable: Yes

  File System: Journaled HFS+

  BSD Name: disk0s2

  Mount Point: /

  Content: Apple_HFS

  Volume UUID: B998D099-F082-3E10-B5C5-D61E96112C43

Boot OS X:

  Capacity: 650 MB (650,002,432 bytes)

  BSD Name: disk0s3

  Content: Apple_Boot

  Volume UUID: D87EC499-83D0-30CE-A872-87E4D0AF1667

Hardware Overview:

  Model Name: Mac Pro

  Model Identifier: MacPro5,1

  Processor Name: Quad-Core Intel Xeon

  Processor Speed: 2.8 GHz

  Number of Processors: 1

  Total Number of Cores: 4

  L2 Cache (per Core): 256 KB

  L3 Cache: 8 MB

  Memory: 32 GB

  Boot ROM Version: MP51.0087.B00

  SMC Version (system): 1.39f11

  SMC Version (processor tray): 1.39f11

  Serial Number (system): H00382U****

  Serial Number (processor tray): J503305LV****     

  Hardware UUID: 2915A197-5F45-54EC-BC82-BC78FA825ABD