Football Manager 2020

Like Pongulf92 said, it is recommended you do this in a country with only 2 divisions as you only need to promote 1 time. Iceland being one of these countries. Some other countries contains B teams from the big teams in that nation, they can't promote so if a B team finishes 1st and you finish 2nd, you will promote instead of them. So I'll list them in case you don't have the editor and have to do this legit. There may be other nations with B teams that I haven't noted down, but these are the ones I know off the top of my head as I have played in these nations before.

The countries that this is the case for is as follows
- Austria (Contains B Teams)
- Belarus
- Bulgaria
- Croatia (Contains B Teams)
- Czech Republic
- Finland
- Greece
- Hungary
- Iceland
- Israel
- Netherlands (Contains B teams)
- Poland
- Republic of Ireland
- Russia (Contains B teams)
- Serbia
- Slovakia
- Slovenia
- Switzerland
- Ukraine
- China
- Indonesia
- Malaysia
- South Korea
- Chile
- Argentina
- Colombia
- Uruguay
- Mexico
- South Africa

My personal recommendation would be Netherlands.

Netherlands contains 4 B teams, so only 16/20 teams can promote. Also 3 teams can promote each year, 1 is automatic (1st place) and then the next 8 non B teams qualify for the promotion playoffs, which the 2 winners go up against the bottom 2 teams of the first division. Last season, a team that finished 8th ended up being promoted. So it is quite forgiving if your best striker gets injured mid season for example. And the Netherlands is one of the best nations for youth prospects, so you could get some good quality on loan.

Iceland has only 2 divisions. Very easy if you use editor and add 200 in skills.