Erase the "db.sqlite3" file. The issue here is that your current data base will be erased, so you will have to remake it again.
Inside the migrations folder of your edited app, erase the last updated file. Remember that the first created file is: "0001_initial.py". For example: I made a new class and register it by the "makemigrations" and "migrate" procedure, now a new file called "0002_auto_etc.py" was created; erase it.
Go to the "pycache" folder (inside the migrations folder) and erase the file "0002_auto_etc.pyc".
Finally, go to the console and use "python manage.py makemigrations" and "python manage.py migrate".