This is the view:
def showProject(request, project_slug):
project = Project.objects.get(slug=project_slug)
tickets = Ticket.objects.filter(project=project)
paylo
project = Project.objects.get(slug=project_slug)
is raising an exception (DoesNotExist), meaning that there is no project in the Project table with a slug corresponding to the value in project_slug
The regex in your urls might be wrong, the link sending you to showProject view might be wrong or there mightn't be a project in the table yet corresponding to that slug