You have to apply for review of the domain or subdomain using Google Search Console as shown in below video.
https://youtu.be/sfuaxxDYbIM
You can include the link to privacy policy in your review and also mention that you are using the self-hosted script bought from CodeCanyon and provide the link to the product page. Privacy policy text is included with the web app. You can find it in root folder where you installed it. Just change the company name, email address and date in privacy policy before you submit the review. It should remove the warning.