I see that you’re using the Amazon S3 PutObject action from this connector, I imagine because your project is in Mendix 9.12.4. I’d like to point out that there’s a platform-supported connector for Amazon S3 that you can use if you have the opportunity to upgrade your project to 9.18.0 or higher which may resolve your issue.
Feel free to reach out to me.
You missed the prerequisite https://marketplace.mendix.com/link/component/120333
Go Make It
Key is the file name that you want to write in to the S3 bucket