AWS OpenSearch Serverless - returning 403 error with correct access policy

I have added those into my IAM but still getting a 403 error.

Heres a snippet of my data access policy: policy = jsonencode([{ "Rules" : [ { "ResourceType" : "index", "Resource" : [ "index/my-data-collection-${var.stage}/*" ], "Permission" : [ "aoss:CreateIndex", "aoss:ReadDocument", "aoss:UpdateIndex", "aoss:WriteDocument", "aoss:*" ]}], "Principal" : [ "arn:${partition}:iam::${account_id}:user/${stage}"]

And a snippet of my IAM: `iamRoleStatements:

• Effect: Allow Action:
• aoss:APIAccessAll Resource:
• arn:aws:aoss:${aws:region}:${aws:accountId}:collection/id`

Is that the full data access policy? If so, you will need to add the ARN of the Lamaba function's execution role to the Principal element.

For example

policy = jsonencode([
{ 
    "Rules" : [{
        "ResourceType" : "index", 
        "Resource" : [ 
            "index/my-data-collection-${var.stage}/*" ], 
            "Permission" : [ 
                    "aoss:CreateIndex",
                    "aoss:ReadDocument",
                    "aoss:UpdateIndex",
                    "aoss:WriteDocument",
                    "aoss:*"
              ]
      }], 
      "Principal" : [
            "arn:${partition}:iam::${account_id}:user/${stage}",
            "arn:${partition}:iam::${account_id}:role/${LambdaExecutionRoleName}"
       ]