You have a NON-NAV APIM which has only 8GB of memory, so you need to make room for any type of modifications by deleting Gracenotes. Once you have that done, the process will likely work. See the tutorial…
Tutorial: Sync 3.4 NON-NAV APIM: Failure to Update to Newer Version Sync 3.4 - Ford / Guides - CyanLabs Official Community
This applies to upgrading or downgrading thru autoinstall. Only perform the Gracenotes deletion and the downgrade of the version.