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You have this error because the user you try to assign is not authenticated.
Add a permission to your view like this :
from rest_framework import permissions
class add_to_cart_view(APIView):
permission_classes = [permissions.IsAuthenticated]
# Rest of the code here
NB : You will need to authenticate the user before make a post request.
UPDATE : If you want to test with non authenticated user.
You can create a generic user for that :
# Create a user with a name unauthenticated_user in the admin
# In the view :
if request.user.is_authenticated:
# Do something for authenticated users.
# Your code previous code here
else:
# retrieve the unauthenticated_user you create
# use your custom user here
Source:stackexchange.com