Use google cloud storage for CTFd upload file directory

Hi, I want to use google cloud storage for CTFd upload file directory. This is my step deploying CTFd :

Firstly, I created vm instance on google cloud platform

Then, I created bucket on google cloud storage named geo-ctfd and create access key for service account. I also made all objects in a bucket publicly readable and set CORS configuration on its bucket

Then, I added this environment variable on the docker-compose.yml (right below reverse_proxy):

After all of that, I ran sudo docker-compose up

Then, CTFd has been successfully deployed. But after I finished making the challenge, an error page appeared, it says “500 An Internal Server Error has occurred”.
“EventSource’s response has a MIME type (“text/html”) that is not “text/event-stream”. Aborting the connection.”

Anybody has a clue ?