rest api gateway in front of dynamodb gives 404 on GET calls

Hi, I am following this guide to create a test REST api gw in front of a dynamo db table.

While the POST call tests work, when I try the GET to retrieve a username I always get a 404

Fri Feb 11 07:17:40 UTC 2022 : Endpoint request URI: https://dynamodb.us-east-1.amazonaws.com/?Action=Query
Fri Feb 11 07:17:40 UTC 2022 : Endpoint request headers: {Authorization=****************************************************************************************************************************************************************************************************************************************************************************************82e823, X-Amz-Date=20220211T071740Z, x-amzn-apigateway-api-id=2xxp28f39k, Accept=application/json, User-Agent=AmazonAPIGateway_2xxp28f39k, X-Amz-Security-Token=xxxxx
Fri Feb 11 07:17:40 UTC 2022 : Endpoint request body after transformations: {
    "TableName": "users",
    "IndexName": "username-index",
    "KeyConditionExpression": "username = :val",
    "ExpressionAttributeValues": {
        ":val": {
            "S": "ABE"
        }
    }
}
Fri Feb 11 07:17:40 UTC 2022 : Sending request to https://dynamodb.us-east-1.amazonaws.com/?Action=Query
Fri Feb 11 07:17:40 UTC 2022 : Received response. Status: 404, Integration latency: 2 ms

this is my GET integration request application/json mapping template

{
    "TableName": "users",
    "IndexName": "username-index",
    "KeyConditionExpression": "username = :val",
    "ExpressionAttributeValues": {
        ":val": {
            "S": "$input.params('username')"
        }
    }
}

If I test the query on dynamodb it works:

aws dynamodb query \
    --table-name $tablename \
    --index-name $indexname \
    --key-condition-expression "username = :v1" \
    --expression-attribute-values file://expression-attributes.json \
    --select ALL_PROJECTED_ATTRIBUTES \
    --return-consumed-capacity INDEXES

where file://expression-attributes.json

{
    ":v1": {"S": "ABE"}
}



gives

{ "Items": [ { "realname": { "S": "xxxxx" }, "username": { "S": "ABE" }, "uid": { "S": "fc1d4fa5-6157-47c7-85a7-3953e477b396" } }

any idea what I am missing?