Some key things to check when adding Hyper-V to Veeam:

• Local Security Policy: Ensure remote access via DCOM/WMI is allowed.

• UAC: Might block remote admin actions — adjust or disable for testing.

• AV/Firewall: Confirm required ports (e.g., TCP 135, 445, dynamic RPC) are open.

• Credentials Format: Use hostname\user when adding credentials.

• User Permissions: The account must be in the local Administrators group.

Also check DNS resolution, time sync, and that WinRM is properly configured. Hope this helps!

Also, you can check that link:
Failed to connect Hyper-V standalone (2012 R2) | Veeam Community Resource Hub

It’s most likely UAC. I’m assuming you’re using a non-domain account and not the built in administrator user? If so it is blocked with default policies. Links from the others above can help resolving this

​@JaroJ Have you tried with the local Administrator?

Also worth checking if Admin$ Share was disabled.

It’s a fairly common security recommendation and based on the error, could see it being as simple as that path really isn’t valid for the HV host since it’s not enabled.

Strongly recommend check ​@Link State’s suggestion as well as ​@MicoolPaul’s suggestion -- Local Administrator account is a must as your first step to help isolate issues when adding HyperV servers, as non-local Administrator accounts have different SID and may have unexpected restrictions.

If you can’t get it, a Support Case is the way to go Support will be able to get a better idea of what’s going on from the logs.

While not exactly the same challenge you are facing, this KB article’s  “Cause, Solution and Use Case Examples” affirm what ​@MicoolPaul, ​@Link State, and ​@ddomask have said about the local administrator account and it’s unique SID being required when a not using a domain account.