Amazon SP-API Access token is missing error

I am getting the following result from an SP-API call using CURL within PHP. It is a private app created in seller central and all permissions have been granted. We are just trying to retrieve our own data.

Array (
    [errors] => Array
        (
            [0] => Array
                (
                    [code] => Unauthorized
                    [message] => Access to requested resource is denied.
                    [details] => Access token is missing in the request header.
                )

        )

)

Access token is retrieved with a refresh token and included, here is the code. amazonApiUrl is https://sellingpartnerapi-na.amazon.com

$headers = array(
            'Content-Type: application/json',
            'Authorization: Bearer '.$accessToken
        );

        $curl = curl_init($this->amazonApiUrl."/catalog/2022-04-01/items?marketplaceIds=ATVPDKIKX0DER");

        curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
        curl_setopt($curl, CURLOPT_USERAGENT, 'XXXXXXXXX-app/v1');

        $response = curl_exec($curl);

The access token exists and has been retrieved from https://api.amazon.com/auth/o2/token#

What can be wrong ?

I was expecting our products list but getting the above error.