Testing applications on ChromeOS sometimes requires a specific version of ChromeOS, it could either be the latest, or a past version. This is how to lock chromebooks to a specific ChromeOS.
Some manufacturers may not have released the version you selected. If that is the case, select a compatible previous version before it. We have experienced that HP is behind, while ChromeOS 79 was listed on the version list, they only had up to version 78, causing the Chromebooks to not update at all.