Request.GET.get() returns None all the time - django

I have these two models :

class Payment(models.Model):
    admin = models.ForeignKey(User,on_delete=models.PROTECT)
    client_seller = models.ForeignKey(ClientCompany,on_delete=models.PROTECT,blank=True)
    next_payment = models.OneToOneField(NextPayment,blank=True,null=True,related_name='next_payments',on_delete=models.PROTECT)
    #others

class NextPayment(models.Model):
    next_payment = models.DateTimeField()
    status = models.BooleanField(default=True)

I want to create NextPayment instance before Payment instance will be create , and assign the NextPayment object to Payment > next_payment field ! here is my views.py

@login_required
def create_payment(request):
    main_form = PaymentForm()
    next_paymentform = NextPaymentForm()
    next_payment= request.GET.get('next_payment')
    print(next_payment)
    if request.method == 'POST' and request.is_ajax():
        main_form = PaymentForm(request.POST)
        next_paymentform = NextPaymentForm(request.POST)
        if main_form.is_valid():
            main_obj = main_form.save(commit=False)
            main_obj.admin = request.user
            if next_payment:
                date_next = next_paymentform(next_payment=next_payment)
                #date_next = NextPayment(next_payment=next_payment,status=True) also tried this
                date_next.save()
                main_obj.next_payment= date_next
                main_obj.save()
            else:
                main_obj.save()
            data = {
                 'id':main_obj.id
            }
            return JsonResponse({'success':True,'data':data})
        else:
            return JsonResponse({'success':False,'error_msg':main_form.errors,'next_paymentform':next_paymentform.errors})
    return render(request,'payments/pay.html',{'main_form':main_form,'next_paymentform':next_paymentform})

my forms.py

class NextPaymentForm(forms.ModelForm):
    next_payment = forms.DateTimeField(input_formats=['%Y-%m-%dT%H:%M'],widget=forms.DateTimeInput(attrs={'type':'datetime-local','class':'form-control'}),required=False)    
    class Meta:
        model = NextPayment
        fields = ['next_payment']

class PaymentForm(forms.ModelForm):
    class Meta:
        model = Payment
        fields = ['client_seller','type_of_payment','price','note']

here is my template

            <form action="" method="POST" id="create-payment-form">{% csrf_token %}        

            <div class="row">
                <div class="col-md-6">
                    <div class="form-group">
                    <i class="fas fa-user-check"></i>
                    <label>name </label>
                    {{ main_form.client_seller | add_class:'form-control' | append_attr:'onchange:currentPaymentBalance();' }}
                    </div>
                </div>
                <!-- /.col -->
                <div class="col-md-6">
                    <div class="form-group">
                    <i class="fas fa-money-check-alt"></i>
                    <label>number</label>
                    <input type="number" class="form-control" disabled id="balance_client">
                    </div>

                    <!-- /.form-group -->
                </div>
                <div class="col-md-6">
                    <div class="form-group">
                    <i class="far fa-handshake"></i>
                    <label> type</label>
                    {{ main_form.type_of_payment | add_class:'form-control' }}
                    </div>
                </div>
                <div class="col-md-6">
                    <div class="form-group">
                    <i class="fas fa-money-check-edit-alt"></i>
                    <label> price</label>
                    {{ main_form.price }}
                    </div>
                </div>    
                <div class="col-md-12 col-12">
                    <div class="form-group text-center ">
                    <i class="far fa-comment-dots"></i>
                    <label class="">note</label> 
                    {{ main_form.note | add_class:'form-control' }}
                    </div>
                </div>

                    <div class="col-md-6 justify-content-center">
                        <div class="form-group">
                        <i class="fas fa-quote-right"></i>
                        <label> next payment</label>
                        {{ next_paymentform.next_payment }}
                        </div>
                    </div>                                        
    
            <!-- /.col -->
            </div>
            <!-- /.row -->
            <div class="card-footer">
                <div class="row justify-content-center">
                    <button type="submit" class="btn btn-lg btn-success">زیادکردن</button>
                </div>
            </div>
        </form>

next_payment prints None! but it wasn't None but doesn't work, and only saves the main form! I need to check if the next_payment exists then create an instance of NextPayment and assign that new instance to the Payment model? any idea is much appreciated.
thank you in advance...

Since your form is sending a POST request you have to change

next_payment= request.GET.get('next_payment')

to

next_payment= request.POST.get('next_payment')
Back to Top