This looks like a dependency/module import issue, not an AWS configuration issue yet.
The important clues are these two parts of the error:
software.amazon.awssdk.services.s3... does not exist and awsauthentication.impl... does not exist. That means Studio Pro cannot find the AWS SDK JARs and also cannot find classes from the AWS Authentication Connector that the Amazon S3 Connector depends on. Mendix documents that the S3 connector requires AWS Authentication, and if you use Amazon S3 Connector 3.0+, it requires AWS Authentication 3.0+.
So the most likely causes are:
For Mendix 11.6, I would do this:
First, remove both modules from the app.
Then open the app directory and clean the userlib folder of old AWS-related JARs.
After that, reimport AWS Authentication Connector first, then Amazon S3 Connector, making sure the versions are compatible with each other.
Finally, do a clean restart of the app. This is the standard fix path when Marketplace Java modules fail because of missing or conflicting libraries.
Also, I would make sure you are using the latest Marketplace versions that support Mendix 11, because Mendix 11.6 introduced AWS S3 SDK v2 changes on the platform side, so staying current with the AWS modules is even more important.
So in short: this is most likely a broken module/library setup in the project, not a problem with your bucket or AWS credentials. Start with version alignment and userlib cleanup.
If this resolves your issue, you can mark it as accepted.