要約

You are unable to create new Jira issues and are getting this error message during the action:

Approver: Pick another user as the selected user doesn't exist.

環境

Jira Cloud

原因

You have an invalid value set for the Approver field.

ソリューション

Looking the Custom Fields configuration from your instance, you will find the Approver field: https://<sitename>.atlassian.net/secure/admin/ViewCustomFields.jspa

If you open the details of the affected custom field, you will note you probably have a default value set. Something like this:

This default unknown value is what is causing the problem here. You need to remove it or, at least, edit it.
To edit or remove this default value, please follow these steps:

1. Go to > Issues;
2. Click on Custom fields on the sidebar;
3. Click on the right next to and Context and default value;
4. Now you have 3 options: edit the default value, remove the entire context or even delete the custom field itself if you want (in case you're not using it at all);
5. Click 'Modify';
6. Now you will be able to create issues again.