While executing Test Run, we are seeing 'The default assignee does not have assignable permission or unassigned issues are turned off' error. To resolve this, follow below steps:

1. Select Project in which you are getting this error. Refer screenshots for steps :
   
   
   
   
2. Go to permissions by following steps in screenshot :
   
3. Grant permission to 'Assignable User' role by the following screenshot :
   
4. Now you will be able to execute Test Run