I want to know why I can’t assign an issue or an alert rule to a team and see the error:

You can only assign teams you are a member of.

That behavior is expected. When Open Membership is turned off, you can only assign issues and alerts to teams that you belong to.

To assign something to another team, an organization Manager, Owner, or Team Admin can add you to that team. Alternatively, an organization admin can enable Open Membership under Settings → General Settings.

If the team doesn't appear in the assignee list at all, see Why can't I assign an issue to the desired team? for additional troubleshooting steps.