Solved

Why i cant go amateur league?

First of all, Support will not manually relegate so you have to work it out yourself.

If you can find a league where they have enough teams in Pro to activate relegation, you can join just before the final race and will be relegated. Then you can re-join your friend's league in the Rookie tier.
The problem is the league system is in a bit of a mess at the moment so finding a league with enough teams to trigger relegation is going to be a challenge.

As an alternative, someone with many bot accounts may offer to pad out your pro tier which will also work but then you would have to spend a season intentionally not scoring any points to ensure relegation.
Problem with this solution is that the current purge on "farming" means there are likely to be fewer people willing to risk using bots for fear of having sanctions applied.