If this EventDefinitionKey is definitely correct, this issue usually occurs if the "Installed Package" (API credentials) doesn't have the correct access rights to the Business Unit your journey is implemented in.
Try checking this by navigating go Administration -> Account -> Installed Packages and clicking on the package you used for the API Integration. Then check the information under Components -> API Integration -> Business Unit (see the screenshot below 1).
In addition to that, also pass the MID of your Business Unit in the account_id
parameter when requesting the access token (accessToken request documentation). That should solve the problem.