no module name.models

I'm practicing from a book Begining django e-commerce and m stuck in one place. My project name is ecomstore, and in it i have two app, one is preview and the other is catalog. In my preview app, there's just html in the templates dir, and thats not the problem, because the result was fine when i ran the server. And then i made a new app, catalog. It has two models, Category and Product. Then to register it with the admin, i made one admin.py file. And there i imported those two module by:
from django.contrib import admin
from django.ecomstore.catalog.models import Category, Product
...and some other forms related import.
Then when i run the server, i get an Import error: No module named catalog.models.
I have installed my app in settings.py.  Hope u guys could solve my problem.

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