Comment 7 for bug 1767137

2.40-beta2 is still in the bionic archive as of now. To get 2.4.0 final you need to enable the ppa:maas/stable -- since it released 2 weeks ago I assume an SRU for this is being worked on but isn't yet finalised?

Otherwise as stated earlier, the workaround for this issue is to just explicitly enable syncing the xenial image on the images tab.