@Krassotkin did you provide a summary with your change? If not, they may be a known issue - can you try again with a summary?

It's not possible to do any content model change from the UI until T252963 is resolved. The edit summary is checked at the initial load of the page (and you can't put any summary before the page opens), so the moment you open it, it will fatal. But maybe it's possible via the api, not sure

In T253441#6159971, @DannyS712 wrote:

@Krassotkin did you provide a summary with your change? If not, they may be a known issue - can you try again with a summary?

Provide a summary - where?
When I try to change content model on any page (the first screenshot link "изменить" - "change"), I immediately get an error message (second screenshot).

https://ru.wikinews.org/w/index.php?title=Участник:Krassotkin/test&action=info

https://ru.wikinews.org/wiki/Служебная:ChangeContentModel/Участник:Krassotkin/test