You need to get more information for this error. Try to get verbose debug or something similar. The rest of the messages may not (and I think do not) be helpful, especially the part about pppd and root.
To increase verbosity, see https://github.com/adrienverge/openfortivpn/issuese, /335
That’s about Crostini which uses LXD. But it does not run proper Linux kernel so it is not possible to make it work there.
Have a look at how to get OpenVPN to work with LXD,
It is likely that you need to do something similar.