Select a valid choice. ["objects"] is not one of the available choices. in Django

I'm going to create a form where the user can select any number of objects from the options.(MultipleChoiceField)

Here are my files:

DAY_CHOICES = (
('saturday', 'شنبه'),
('sunday', 'یکشنبه'),
('monday', 'دوشنبه'),
('tuesday', 'سه شنبه'),
('wednesday', 'چهارشنبه'),
('thursday', 'پنج شنبه'),
('friday', 'جمعه'),)

# Models.py
class CorporationRequest(models.Model):
    user = models.ForeignKey(User)
    title = models.CharField(max_length=250, )
    explain = models.CharField(max_length=500, )
    assistance = models.CharField(max_length=250, choices=ASSISTANCE_CHOICES)
    days =   # MultipleChoiceField
    created_date = models.DateTimeField( auto_now_add=True)

# Forms.py
class ObjectsForm(ModelForm):
    objects = forms.MultipleChoiceField(widget=forms.CheckboxSelectMultiple,
                                     choices=OBJECT_CHOICES)
    class Meta:
        model = ObjectModel
        fields = '__all__'
# Views.py 
def add_corporation(request)
    if request.method == "POST":
        form = ObjectsForm(request.POST)
        if form.is_valid():
            new_corporation = form.save(commit=False)
            new_corporation.days = ','.join(form.cleaned_data['days'])
            new_corporation.save()
            return redirect('administrator:view_admin_user_corporation')
    else:
        form = ObjectsForm()
    template = 'corporation.html'
    context = {'form': form, }
    return render(request, template, context)

When I click on the submit button (for example select object 1 and 2), I get this error:

Select a valid choice. ['1', '2'] is not one of the available choices.-

Hint: each user send request to us that contain explanation and days that he can do something. So, Each user can select any day that he want.

If you must leave CorporationRequest.days as a CharField then you can save the day choices as a comma separated string in the field

class CorporationRequest(models.Model):
    days = models.CharField(max_length=255)

You have to clean the data coming from the multiple choice field so that it can be saved in the field

class CorporationRequestForm(ModelForm):
    days = forms.MultipleChoiceField(widget=forms.CheckboxSelectMultiple,
                                 choices=DAY_CHOICES)
    class Meta:
        model = CorporationRequest
        fields = '__all__'

    def clean_days(self):
        return ','.join(self.cleaned_data['days'])

This will mean that CorporationRequest.days is now a string and not a list of days. You could add a method to the CorporationRequest model to return the list of days

def get_days(self):
    return self.days.split(',')