We are successfully using the Legacy HTTP Server Protocol on our server for FCM. I wanted to update to FCM HTTP v1 API today.
I did it step by step and when the serv
I had this problem, and it seems you need to enable FCM API for your project at Google API console.
I was having this issue of getting a 403 from the FCM HTTP v1 API. I had FCM Messaging API enabled, but my problem was that the service account I was using didn't have the correct role to make requests to the FCM API. You can try creating a new service account with the "project Owner" role to see if that helps.