I am trying to create a webhook with topic INVENTORY_LEVELS/UPDATE but I am getting the error "Cannot read properties of undefined (reading 'webhookSubscriptions')" at the creation. I've already checked my app scopes and I have the read_inventory scope.
If so, and this issue has persisted, I wanted to ask a few more questions.
Have you worked through any debugging with an API client? I was able to register a subscription with both a REST post request and GraphQL mutation, neither has issues when the app had the correct scope access (read_inventory).
Can you register other subscriptions with the same app?
Beyond that, if you are able to replicate this behaviour and can log an x-request-id response header value returned with the error, I can absolutely investigate logs on our end, pass back insights, suggestions, or next steps!