Step 6 - Add App Clip associated domains

This is a standard step for App Clip development.
See the official Apple documentation.

6.1

Open the <app clip target>.entitlements file.
Add an Associated Domains Array type.
Add a row to the array with appclips:<your bundle id>.

6.2

The same associated domains entitlement needs to be added
to your main app, as well.

Copy the <app clip target>.entitlements file from your
App Clip group to your main app group and rename it to
the same name as your main target
such as Runner.entitlements.

Open the file and delete the
Parent Application Identifiers
entry for the main app’s entitlement file
(leave that entry for the App Clip’s entitlement file).

6.3