Why does Google Merchant Center say my Shopify products are missing a brand?

Short answer

Google reads the brand from your product data, and Shopify's Google & YouTube channel takes it from the product's Vendor field. If Vendor is empty, the product shows under "Needs attention" as missing a brand. Enter the manufacturer's brand in Vendor, or your store name if you make the product yourself.

What Google accepts as a brand

The manufacturer's brand name.
Your store's name, if you make the product yourself.
The manufacturer or supplier name, if the product has no brand.

Don't use the store name for a product someone else makes and brands. Don't use "N/A", "Generic" or "Unbranded" when there is a real brand.

Where it's required

Brand is required for all products in Apparel & Accessories. For shoes, sunglasses, handbags and watches, Google also expects a GTIN or MPN. Meta's catalog requires a brand on every item, so an empty Vendor field also hurts on Facebook and Instagram Shopping.

How to fix it on Shopify

Products → filter by Vendor, and look for empty or placeholder values (your store's default, "Default", the supplier's warehouse name).
For one product: open it, set Vendor on the right under Product organization, and save.
For many: select them, then Bulk edit, add the Vendor column and fill it. Or export a CSV, fill the Vendor column and re-import.
Check that the Google & YouTube channel shows the products as synced. Merchant Center clears the issue after its next check.

Common Shopify causes

Products created by a dropshipping or import app that left Vendor empty or set it to the supplier's code.
Vendor set to the store name for resold brands, which isn't wrong for Google's check but hides the real brand from shoppers who search for it.

Questions

Is my store name a valid brand?

Yes, for products you make yourself. For products made by another brand, use that brand.