S3 gateway endpoint unable to upload file to S3

What is the error you are getting when you fail to upload the file to the bucket?

If it's a Forbidden 403 status code error, its a policy issue (could be either EC2 instance role's policy, endpoint policy or bucket policy).

If it's a timeout it can be missing route or missing outbound security group rule associated with the EC2 instance.

You can work backwards from the error message you are getting. Here is the re:post link - https://repost.aws/knowledge-center/connect-s3-vpc-endpoint that you can refer to, it gives a detailed troubleshooting steps for this scenario.