This error indicates that the task was not assigned to any users and usually depends on how the task was configured.  If it was configured to assign the task to a workflow variable, SmartObject field, or item reference and that value was empty, null, blank; then it would throw this error because there was not a destination that it can assign it to.

If applicable, you can try populating this empty field and retrying, otherwise you may need to do a GoToActivity back to a previous step and make sure that the empty field is populated.

If populated via form, make sure you use validation to prevent empty fields, if that field will be used for this task assignment.

Or in the workflow flow check to see if it is empty first before the task assignment, and handle the case when it is empty vs when it is not.

Please provide more detail on which user is being assigned the task and how K2 is providing this. The error means that no user was provided, so the issue lies with the task assignment