You absolutely MUST use Ubuntu 18.04 LTS server edition as the underlying operating system to install Mail-in-a-Box. So there is no point in trying to investigate the error you have encountered. Please do a reinstall of your VPS’s OS and try again.
No, not at all. Gandi’s system is rather good, so assuming that this is the first time you have set Glue records for the domain, they should be functional near immediately.