what dan I do wit h error: Response code 403 (Forbidden)

It is just an authentication issue. Indeed I changed my password on this sandbox a few days ago. It appears that the OAuth credential is therefore wrong.

Go to Setup > Named Credentials.

Find the one related to your sandbox.
Delete the entry having your name.
Return to Devops Center and on the environment click on the link to the sandbox. You can now + connect and enter the new password.
Now the promotion can start.

I don't say that it is the same issue for everyone, but it resolved my 403 issue.

[Edit : note that I was still able to open the environment from Devops Center!]

Response code 403 (Forbidden)