Hello Jainesh,

Thank you for looking into this! Unfortunately It didn't work, but I am sure it is because I am doing something wrong because I don't know coding or phyton. (I can't send screenshots because I don't have karma, so I will list below what I entered in all fields: ("entrega final" is the name of the stage all tasks are moved to/project consequently needs to be moved to)

Model: Project
trigger: Update
Activation fields: Task stage (project.project)

Before updating the domain: (I didn't touch this field)
Apply to: (I didn't touch this field)

Action to do: execute phyton code

Phyton code:
1.if len(records.project_id.task_ids.mapped('stage_id.name'))==1:
2.   stage_id = env['project.project.stage'].search([('name','=','entrega final')])
3.   records.project_id.write({"stage_id":stage_id})

(I left 3 spaces as "indent" in the second 2 lines)

Thank you very much for all your help!

Hello Jainesh,
please disregard my previous comment, I had the Model wrong :(,
We corrected the model and now the code did work in a part: what happens now is that when all tasks are moved to the stage "entrega final" the project is moved to a non-existing stage that is created by the system :/

could you please help us with this?

This is the correct answer! The code did work, we were placing it incorrectly. Thank you very much for all your help!