How to enable and make sure an issue link named "Relates" is available in the project screen

Looking at the error message, it appears that you'll need to turn on issue linking for that particular project. Have you tired enabling this feature and are you still getting the error? 

Here's the documentation on how to enable this feature and what permissions are needed for issue linking - https://support.atlassian.com/jira-cloud-administration/docs/configure-issue-linking/