From the first line of the text in your screen dump:
For this to work, you will need rclone available on a machine that has a web browser available.
So you need to have rclone on your Windows pc (or some other pc with a web browser) to get the token for your proxmox server. You can download it from here: Rclone downloads
How do you understand this: "Affinché funzioni, avrai bisogno di rclone su una macchina con un browser web disponibile"?
There seems to be an issue with your own Client ID. I suggest we first get your proxmox running with the default rclone Client ID and then fix your own Client ID next.
Try removing the client_id and client_secret from your config
then redo the entire authorization flow 1) starting on proxmox, 2) authorizing on your ubunto desktop and then 3) completing/pasting the token to your proxmox