I had the same problem and I have found the solution.
In the documentation, you can find this condition:
"You can delete a platform if a build has never been uploaded for the platform you wish to delete. Additionally, at least one existing platform version needs to be in an editable app status."
So I have created a news editing version for my iOS app and then I could remove the macOS version of my app.
I hope this will help you correct your problem
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Hello,
You need to use two endpoints (appstoreconnectapi)
Endpoint 1 : https://developer.apple.com/documentation/appstoreconnectapi/list_all_price_points_for_an_in-app_purchase
This entry point allows you to retrieve the inAppPurchasePricePoint ID for a specific tier of your purchase .
Example: https://api.appstoreconnect.apple.com/v2/inAppPurchases/{Purchase_ID}/pricePoints?limit=200&filter[territory]=USA (GET request)
Endpoint 2 : https://developer.apple.com/documentation/appstoreconnectapi/add_a_scheduled_price_change_to_an_in-app_purchase
This entry point allows you to add the new tier by specifying the inAppPurchasePricePointID in included (https://developer.apple.com/documentation/appstoreconnectapi/inapppurchasepriceinlinecreate/relationships/inapppurchasepricepoint/data)