When VAO tries to deploy a VAO agent to a standalone Veeam Backup & Replication server, you get a generic error "Failed to communicate to agent", and all VAO attempts to communicate with the VAO agent fail. You can also see the following error in the VAO agent logs (located in the log folder under %programdata%\Veeam\Veeam Availability Orchestrator\Log) on the Veeam Backup & Replication server:

The permissions on the log folder are ordered incorrectly. To check if this is the issue, open the properties of the folder %programdata%\Veeam\Availability Orchestrator and switch to the Security tab. If you get the error "The permissions on <folder> are incorrectly ordered, which may cause some entries to be ineffective", you must reorder the permissions.

Open the properties of the parent folder %programdata%\Veeam on the Veeam Backup & Replication server,  switch to the Security tab, click Advanced and select the Replace permissions entries on all child objects with entries shown here that apply to child objects option.