We need to temporarily block the 'Existing' auto-pinning policy until we make it more usable (see bz 1957551 and bz 1957526)
verified on ovirt-engine-4.4.7-0.23.el8ev.noarch. Post action is responded with 2021-06-02 12:53:54,993+03 ERROR [org.ovirt.engine.api.restapi.resource.AbstractBackendResource] (default task-59) [] Operation Failed: [Cannot add VM. The Auto Pinning policy Existing is currently unsupported.]
This bugzilla is included in oVirt 4.4.7 release, published on July 6th 2021. Since the problem described in this bug report should be resolved in oVirt 4.4.7 release, it has been closed with a resolution of CURRENT RELEASE. If the solution does not work for you, please open a new bug report.