The issue may occur when migrating from one org to another or when starting new in a sandbox. Mainly the error is caused due to the Salesforce Profile Settings.
1. Go to your User Profile 'System administrator', 'Marketing User' etc.
2. Go to Custom Record-Type Settings.
3. Change the Record Type Setting of QA Assignment Group from 'Master' to 'Default Record Type', as shown below:
4. If the record is already set to 'Default Record Type' please change it to '--Master--' and then again change it back to 'Default record type'.
5. This will allow you to create the Assignment Groups successfully.
Please contact us at support@ortooapps.com for any questions.