出现此错误的原因是，您在两个不同的模型（Level和ChildClass）中定义了两个ManyToManyField关系，并且试图对这两个关系使用相同的中介表“level_class”。
在多对多关系中，应该只在一端定义关系，另一端可以通过related_name属性访问它。
在您的示例中，可以从ChildClass模型中删除ManyToManyField，而只将其保留在Level模型中。
试试这个：

#levels/models.py

from django.db import models
from child_class.models import ChildClass

class Level(models.Model):
    name = models.CharField(max_length=50)
    classes = models.ManyToManyField(ChildClass, related_name='levels', blank=True)

#child_class/models.py
from django.db import models

class ChildClass(models.Model):
    name = models.CharField(max_length=50)

According to the Django documentation, https://docs.djangoproject.com/en/4.2/topics/db/models/, under many to many relationship you can create an intermediary model to Foreign key both the Level and Class models to hold the fields that are common and govern both the models. Also the many to many relationship can only be defined on one model not both and it is recommended you put the relationship on a model that brings sense.

class Level(models.Model):
    name = models.CharField(max_length=128

class ChildClass(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Level, through="Intermediate")

class Intermediate(models.Model):
    Level= models.ForeignKey(Level, on_delete=models.CASCADE)
    ChildClass= models.ForeignKey(ChildClass, on_delete=models.CASCADE)