1. Open BeautyPlus, tap BeautyPlus Premium on "Home" as below, choose your preferred plan and continue.
2. Open BeautyPlus, tap "My" and enter Settings in the top right corner, then tap BeautyPlus Premium, choose your preferred plan and continue.
If you fail to purchase, please make sure you downloaded the app from the App Store or Google Play Store, rather than any third party sources.
To resolve payment issues, please refer to the following article.
Fix payment issues on your account
You may also check if you have disabled In-app Purchases on your phone with the following steps.
Settings >> Screen Time >> Content & Privacy Restrictions >> iTunes & App Store Purchases >> In-app Purchases (please set it to Allow)