The web resource adx_annotations/adx.annotations.html is included with Power Pages installations that leverage the file attachment feature with Azure Blob Storage. If it's not visible, please check the following steps:

1. Verify Installation: Ensure that the required Power Pages solution and its dependencies are installed correctly in your environment. Check for any pending updates in the Power Platform Admin Center.
2. Search for the Web Resource:Navigate to Advanced Settings > Customizations > Customize the System > Web Resources. Look for adx_annotations/adx.annotations.html.

If it's missing, it may not have been included during installation, or you might need to reinstall/repair the solution.