You may receive the “403:denied_by_security_policy:create_denied” for the following reasons.

1. The Event or the channel you are trying to subscribe doesn’t exist in the org. Check the username and check whether the Event or channel exists in the org.

2. Use the correct api name of the event.

Syntax:

/event/API_Name_Of_The_Event

Example:

/event/Test_Event__e

3. Check the Profile of the integration user. The profile should have Read and Create access to the Event or Channel.