Moderator: Moved to VMware Fusion
VMware Fusion 8.5.10 is end of life and not supported on macOS Catalina.
It has a number of unpatched security issues and known and unknown problems with macOS Catalina.
It can work, but is not guaranteed to be stable, so it would be recommended to make regular backups of your VMs (NOT with Time Machine)
So my recommendation is to upgrade to VMware Fusion 11.5.1 for which a 30 day trial can be downloaded from:
If OTOH you insist on trying to get 8.5.10 to work then that is possible.
You would start by allowing VMware on the security & privacy general pane (to fix the "Could not open /dev/vmmon" problem)
There's a number of steps you would have to do after that including installing another version in order to get passed all the security questions.
More details at my post here (but read the rest of the thread for context):
Wil| Author of Vimalin. The virtual machine Backup app for VMware Desktop Products
| Vimalin : Automated backups for VMware Fusion and VMware Workstation Professional
| More info at https://www.vimalin.com
| Twitter @wilva
| VMware Wiki at http://www.vi-toolkit.com