The Jira app tries to create the application link automatically.

In same cases, this fails and you’ll see an error on the application link. This is caused if the application link already exists.

There is already an application link with either the same URL or the same consumer key.

Please check your current application links on the Admin area: Applications Application Link
See if there are any links that are called similar to “Jira for Outlook” or “Outlook Email for Jira” and remove them.