That's an error returned by AWS. From experience, it means that the request being sent to them contains one or more attributes that are not supported on the AWS SSO side.
The best way to handle this is to add the AWS Single Sign-On application from the Azure AD Enterprise App Gallery. The provisioning configuration included with this has been configured to be compatible with AWS Single Sign-On's SCIM implementation out of the gate. I would wager that you've configured SCIM provisioning to AWS Single Sign-On via our Custom Non-Gallery Application option instead, which is a "one size fits all" approach that sometimes requires tweaking to get working, as you've seen here.