fix validation error

tournaments/views.py

@@ -63,6 +63,7 @@ from django.core.mail import EmailMessage
 from django.views.decorators.csrf import csrf_protect
 from .services.tournament_registration import TournamentRegistrationService
 from .services.tournament_unregistration import TournamentUnregistrationService
+from django.core.exceptions import ValidationError
 
 
 # Local application imports