[Django]-Could not parse the remainder:

It’s about Template Language.

Variables look like this: {{ variable }}.

Filters look like this: {{ name|lower }}.

Tags look like this: {% tag %}.

To comment-out part of a line in a template, use the comment syntax:
{# #}

There is no such thing as {{% %}}

So, instead of

{{% for myitem in cate.items_by_category.all %}}

You should use

{% for myitem in cate.items_by_category.all %}

Reference:
https://docs.djangoproject.com/en/3.0/ref/templates/language/

The template tags are sourrounded with {% … %}, not {{% … %}}. So you should rewrite the template to:

{% extends 'orders/base.html' %}
{% load static %}

{% block title %}
Menu
{% endblock %}

{% block body %}

<ul>
{% for cate in Categories %}
    <li>{{ cate.category_name }}</li>
    <ul>
        {% for myitem in cate.items_by_category.all %}
            <li>{{ myitem.item_name }}</li>
        {% endfor %}
    </ul>
{% endfor %}
</ul>