In most cases it means, that the Add-On is unable to access instance issues to perform search on them. Usually, this is caused by having specific permissions schemes.
If you have non-default permissions schema, you must make sure that the Add-On's system user has enough permissions to view issues.
For more information please refer to this page:
https://confluence.atlassian.com/display/Cloud/Managing+Users#ManagingUsers-UserscreatedbyAtlassianadd-ons
Similarly, if you've modified default permission schemes for projects and issues, you may need to add access for add-on users. You can add atlassian-addons-project-access to add-ons can perform as designed. To do this, navigate to your administration console and click Issues > Permission schemes. Click the name of your scheme, and under the Operations column, click Add and enter atlassian-addons-project-access from the Project Role drop-down.