Yes, you only have 400MB of memory free, which will not allow Discourse to rebuild. Try bumping it up to 4/5gb, or shutting down the forum before rebuilding and try again.
Assuming that the 400mb free is at idle. Rebuilding is pretty intensive on the system (I’d look it up in Proxmox for my site, but it’s been offline for a bit now), and requires more memory than idling.