ValueError: External ID not found in the system: website.assets_frontend

Solution 1:

I figured it out. Odoo V15 no longer has a template override so you do not need to create an assets.xml file, you just have to include your CSS, JS, etc in the assets bundle in __manifest__.py.

My new manifest.py

{
    'name': "Website Profile SCSS Customization",
    'version': '1.0',
    'depends': ['website_profile'],
    'author': "ES",
    'category': 'Category',
        'description': """Customize SCSS for Website Profile (for e-learning module) to match our branding""",
    'assets': {
        'web.assets_frontend': [          
            'website_profile_scss_customization/static/src/scss/website_profile.scss'
        ]
    }
}