When attempting to install an add-on for JIRA on an existing cloud site you see the following message

Were upgrading our cloud infrastructure to make your Atlassian experience even better. Because of this change, add-ons are not available for your site right now. Please note that this process may take a few weeks.

If you've come across this messaging, watch this ticket to be updated when add-ons will be available to be added to your existing site