All Facebook advertisers are now required to get their domain verified by Facebook - due to changes done by Facebook in preparation for Apple's iOS 14 update.
Please follow these steps to verify your Shopify store domain.
1. Log in to your Facebook business manager -> brand safety -> domains
2. Click 'Add' & add your store's domain
3. Copy the verification tag
4. Log in to your Shopify admin -> online store -> Themes -> Customize
5. Theme actions -> edit code
6. Layout -> theme.liquid
7. Search for </head> (CTRL+F: </head>) and copy the Facebook verfication code right above it -> click 'Save'
8. Go back to the Facebook business manager -> brand safety -> domains -> click 'verify'