Why i am getting this errors while using rspec to test models in ruby and rails?

1) RfidT is not valid without device_mac 
     Failure/Error: expect(obj2.errors[:device_mac]).to include ("not valid without device_mac!")
       expected [] to include "not valid without device_mac!"
     # ./spec/models/rfid_t_spec.rb:20:in `block (2 levels) in <top (required)>'

  2) RfidT  duplicate values of epc are not allowed
     Failure/Error: expect(obj3.errors[:epc]).to include("duplicate values of epc are not allowed!")
       expected [] to include "duplicate values of epc are not allowed!"
     # ./spec/models/rfid_t_spec.rb:27:in `block (2 levels) in <top (required)>'

  3) RfidT is not valid without first_seen 
     Failure/Error: expect(obj4.errors[:first_seen]).to include("cannot be Blank!")
       expected ["can't be blank"] to include "cannot be Blank!"
     # ./spec/models/rfid_t_spec.rb:33:in `block (2 levels) in <top (required)>'

  4) RfidT is not valid without last_seen 
     Failure/Error: expect(obj5.errors[:last_seen]).to include("cannot be Blank!")
       expected ["can't be blank"] to include "cannot be Blank!"
     # ./spec/models/rfid_t_spec.rb:39:in `block (2 levels) in <top (required)>'

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6 examples, 4 failures

But when i am using to_not in place of to in spec file then i am not getting any error .Please tell me what is meaning of this and what is the diffrence between to and to_not in ruby and rails.

my spec file generating no error is:

require 'rails_helper'

RSpec.describe RfidT, type: :model do
   it "is valid without epc,device_mac,first_seen,last_seen,count" do
             obj=RfidT.new(epc:"abc",device_mac:"pqrs",first_seen:10,last_seen:20,count:200)
             expect(obj).to be_valid
 end

 it "is not valid without epc" do
                      obj1=RfidT.new(epc:nil)
                      obj1.valid?
                      #expect(obj1.errors[:epc]).to include("cannot be Blank!")
                      expect(obj1).to_not be_valid

 end

 it"is not valid without device_mac " do
             obj2=RfidT.new(device_mac:nil)
             obj2.valid?
             expect(obj2.errors[:device_mac]).to_not include ("not valid without device_mac!")
 end

 it" duplicate values of epc are not allowed" do
                       RfidT.create(epc:"abc",device_mac:"pqrs",first_seen:10,last_seen:20,count:200)
                       obj3=RfidT.new(epc:"abc",device_mac:"pqrs",first_seen:10,last_seen:20,count:200)
                       obj3.valid?
                       expect(obj3.errors[:epc]).to_not include("duplicate values of epc are not allowed!")

 end
 it "is not valid without first_seen " do
             obj4=RfidT.new(first_seen:nil)
                      obj4.valid?
                      expect(obj4.errors[:first_seen]).not_to include("cannot be Blank!")
 end

  it "is not valid without last_seen " do
             obj5=RfidT.new(last_seen:nil)
                      obj5.valid?
                      expect(obj5.errors[:last_seen]).to_not include("cannot be Blank!")
 end
end