If a country qualifies for the Under 20s World Cup, via the Under 19s Euros, you won't be able to apply for the Under 20s job.
This doesn't matter if the role had a manager who you remove via the Editor.
If the U19s role is available, the advert will say they're 'currently in the FIFA World Cup U-20', but taking this role won't allow you to manager the U20s.
Indeed, the role will continue on without a manager forvever.
The way you can manage the U20s now, is to to talk do so via taking the main national job, and controlling the team.
This bug has been around in at least Football Manager 2018. Disappointed to see it's still present in FM2020.