Confirm the following first:
1. Installed 5.9.1 install file (all defender versions the same)
2. Windows PowerShell 3.0
3. Add-PSSnapin OneIdentity.Defender.AdminTools
4. C:\Program Files\One Identity\Defender. The Management Shell has the defender.models.dll
Solution: Version mismatch of the Defender.Models.dll on 5.9.1. This issue is addressed if perform upgrade Defender 5.9.3 release.