In RepU, open your widget and click Copy next to the embed snippet.
In Shopify, go to Online Store, click the ... button on your current theme, and choose Edit code. Open theme.liquid, find the closing </body> tag, and paste your code just above it.
Click Save, then open your live store to see your Google reviews.
Check your live store in a private window, not the theme editor. A domain-locked widget only shows on your own domain.
Make sure you edited your live theme, not a copy. If you update or change your theme later, add the code again.