You can use the Google Lighthouse to test your Shopify PWA, learn more: https://www.youtube.com/watch?v=WqQ9cc8ek9w
Also you can test your APP using an Android device. Just open your store on your Chrome for Android and navigate. You must see the app install banner soon.
First, make sure you have published your updated theme with the manifest code snippet tag. This code snippet need to be inserted inside of your theme head tag, in the line below the.
Then install test your site with Google Chrome Lighthouse Extension. The score for the progressive web app section needs to be between 80 and 100.
Also, check if the user can be prompted to install the web app on Progressive Web App Passed Audits section.
The app is still not working
Follow this video and check if the Litefy App Proxy URL is yourstore.com/apps/litefy