Import users is failed with no reason

I did user import but it is failed and response is just below

Fail to import clients belongs to company(xxxx). Response from Auth0 is: {"type"=>"users_import", "status"=>"completed", "connection_id"=>"xxx", "summary"=>{"failed"=>1, "updated"=>0, "inserted"=>0, "total"=>1}, "connection"=>"Username-Password-Authentication", "created_at"=>"xxx", "id"=>"xxx"}

I want to know why it is failed.