I'm not clear on the error or the failure location. Questions:
1. Have you followed the deployment guidelines as laid out in the Data Fabric Solution for Cloud Backup workflow guide? This document was just updated this week to provide a little more clarity on the deployment steps. Ensuring that you have correctly configured and enabled SnapCenter components is critical to ensuring the success of the solution. AltaVault (by itself) is not very install or configuration heavy, and should be relatively easy to set up.
2. Where is the error you're receiving coming from - SnapCenter, during the attempt to add a storage connection to AltaVault? If yes, then ensure that the requisite steps to enable SC support are done in AltaVault as per the workflow guide. Remember you have to enable SnapCenter access after you enable the SnapMirror service, as well as add a role based account for SC access within AltaVault's GUI pages. This is done before you attempt to add a storage connection in SC.