Hello guys, for everyone who still facing this problem, I found out that for sandbox testing purchases you need sandbox account, not testflight apple id account. To create it you need to go to your App Store Connect account, go to "Users and Access" and select "Sandbox Testers" tab. Add sandbox account using just dummy name, email, password, country etc. After that, go to your device Settings -> App Store -> At the bottom of this page you'll have Sandbox Account like "Apple ID: $yourCurrentAppleId." Sign out from it, then add your previously added sandbox apple id (enter your email and password) and that's it! Thanks for very clear instructions and user friendly interface from Apple (not really)
tashpulatov00
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