In Apple's "In-App Purchase Programming Guide" I read following advice when it comes to testing in-app purchase using Sandbox test.
Create a test user account in iTunes Connect.
On a development iOS device, sign out of the App Store in Settings.
Then build and run your app from Xcode.
Use your app to make an in-app purchase. When prompted to sign in to the App Store, use your test account.
Note that the text “[Environment: Sandbox]” appears as part of the prompt, indicating that you’re connected to the test environment. If the text “[Environment: Sandbox]” doesn’t appear, you’re using the production environment. Make sure you’re running a development-signed build of your app. Production-signed builds use the production environment.
My question is, How do I make sure that I am running a development-signed build of my app? Where exactly is that done, and what steps must I perform?
Project Setting -- >Build Settings then search "code signing" then you could see the options to select