Unable to install Jira add-on

When trying to install the Jira add-on I am getting the following error:

Error

The add-on failed to be installed: Error calling uninstalled at URL https://xxx.atlassian.net/rest/aci/1.0/installation/jira-bitbucket-connector-plugin/uninstall: 401 Client Error: Unauthorized for url: https://xxx.atlassian.net/rest/aci/1.0/installation/jira-bitbucket-connector-plugin/uninstall

Nothing I can do seems to change it. The add-on is not showing in Jira, and I have admin access to Bitbucket & Jira.