The "SKU not found" error in TestFlight usually means Apple does not recognize your subscription in App Store Connect. This is a configuration issue rather than a workflow problem.
Please check the following:
The Paid Applications Agreement is active in App Store Connect > Business > Agreements
Banking and tax information are completed
Your subscription has pricing, localization, and is part of a subscription group
The Product ID (SKU) matches exactly (case-sensitive) what your app is calling
No required metadata is missing
After making any changes, wait 5–30 minutes, reinstall the TestFlight build, and test again.