RAID 5 showing "degraded, needs rebuilding"

Two possible reasons:
1) the SoftRAID 5.8.3 driver is not actually loading. You can test this by running this command in terminal:
sudo kextstat -b com.softraid.driver.SoftRAID

2. The volume is not mounted or is not mounted read write.
Does your volume mount? Did OS X give a warning that it is damaged and mounted read only?

Two possible reasons:
1) the SoftRAID 5.8.3 driver is not actually loading. You can test this by running this command in terminal:
sudo kextstat -b com.softraid.driver.SoftRAID

2. The volume is not mounted or is not mounted read write.
Does your volume mount? Did OS X give a warning that it is damaged and mounted read only?

1. This was the Output:
Index Refs Address Size Wired Name (Version) UUID
121 0 0xffffff7f81499000 0x3b000 0x3b000 com.softraid.driver.SoftRAID (5.6.8) 84388616-7C03-32BE-8AF7-82AA956DC057
protfp@TFP-Mac-Pro ~ %

2. Drive does not mount. Tried reinstalling SoftRaid Pro XT, but it fails.

You need to disable Secure Boot:
https://srforums.wpengine.com/pages/support/faq/faq_imacpro_secureboot.html

And then run the command again. If your volume is not mounted and 5.8.3 is loading, then it may be a directory issue here.

Disabling worked, 5.8.3 loaded and can see the drive but can't mount. It says it's degraded and failed, it looks like the only issue is that a drive is out of sync.

No, if a volume is not mounting, it cannot rebuild.

Either you do not yet have the driver loaded, or the volume is unmountable. (damaged directory)
In the expanded volume tile, does SoftRAID show a number for Disk Identifier? (or is it a -)

Yesterday, it showed as disk9 but after the removal and reinstall of softRAID, today it shows "-"

Additionally, it won't boot. I've gone into recovery mode, physically disconnected the RAID and first aid'd the boot drives. Still no boot on restart. Planning on wiping the drive and reinstalling OS. Thoughts before I proceed?

A - means either the SoftRAID driver is not loaded (blocked by OS X), or the volume does not have enough information to be "published". Unless the volume has a very seriously damaged volume header, it should have a disk number.

Run this kextache command to make sure the driver is loaded (5.8.3)
Else, you need to run through these steps again:
https://srforums.wpengine.com/pages/support/5.8_Volumes_Not_Mounting.pdf

What will not boot? The computers normal startup volume?
If you startup in "verbose" mode (command v), what is the last line when it hangs?
If you start up in internet recovery mode and reinstall the OS (especially Catalina) it will not harm your existing files and should fix the startup problem.

A - means either the SoftRAID driver is not loaded (blocked by OS X), or the volume does not have enough information to be "published". Unless the volume has a very seriously damaged volume header, it should have a disk number.

Run this kextache command to make sure the driver is loaded (5.8.3)
Else, you need to run through these steps again:
https://srforums.wpengine.com/pages/support/5.8_Volumes_Not_Mounting.pdf

What will not boot? The computers normal startup volume?
If you startup in "verbose" mode (command v), what is the last line when it hangs?
If you start up in internet recovery mode and reinstall the OS (especially Catalina) it will not harm your existing files and should fix the startup problem.

Reinstalling now. Will update . . .

1. Reinstalled 10.5.4
2. Installed SoftRAID XT 5.8.3 (and driver looks to be correctly instaled)
3. Connected RAID5
-RAID5 does not mount
-RAID5 listed as disk8
thoughts?

Update #2:
Useded Diskwarrior to rebuild directory. SUCCESS!
Mounted drive, rebuilding (for the next six months, 22TB)

Thanks for all of your help. If you have any other adivce, please let me know.

Run Disk Warrior monthly. Good practice.

If you get a "disk#" then you have everything working. The problem is probably a damaged directory.

Do you have access to Disk Warrior? You may need it to repair the directory.