I just upgraded a C9300 switch to 17.15.3.1 and I have the same issue:
Welcome to the interactive CLI IOS XE terminal
Verifying your device configuration. Please wait...Done
Unable to launch Configuration Mode due to the following reasons:
1. The device configuration is out of sync with cloud, please check the alert
2. Telemetry subscription 10002 missing or misconfigured, e.g., no 'update-policy'
You are in Read-only Mode
Establishing connection to your device. Please wait...
Connection established successfully
So I'm guessing it is a bug.
I have checked the running config and no telemetry subscriptions are available.
Have you contacted support yet?