Hello Vincent,
The problem you returned to you happens when you are not an Administrator on one of both applications (JIRA or Slack) and try to enable the Slack Integration into JIRA.
As you can see in the article below, the Global Administration Permission is required on both applications:
Can you please check if that's the case?
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.