Hi @KoalaJulien 
Can you share with us the origin of your environments like below.
Here as you can see I have 4 environments.
Development > Staging > Preprod > Prod.
I can create branch only on my Development environment.
And after that, if I want to see the changes on Staging environment, I must use forest push.
The changes are now on Staging.
If you want to see the changes on Preprod you must use forest push with FOREST_ENV_SECRET of your Preprod environment.
As you can see on this page of documentation Forest CLI commands - Developer guide.
You can read more about push command by reading this push - Developer guide
Be careful the usage of deploy command can work to put changes from Preprod to Production.
You can read more here https://docs.forestadmin.com/documentation/reference-guide/how-it-works/developing-on-forest-admin/forest-cli-commands/deploy
Let me know if that help 