Error Exception Array to String Conversion in Curl

Hi I'm trying to to Integrate Hubtel Payment Gateway but getting Array to string Conversion Error Exception .

if i do CURLOPT_POSTFIELDS =>http_build_query ($invoice), im getting blank response

please help

     $items = ["name" => "Harry Potter", "quantity" => 1, "unitPrice" => 50];
      $invoice = array("items" => $items, "totalAmount" => 1, "description" => "test", "callbackUrl" => "https://localhost:8000/status","returnUrl"=>"https://localhost:8000/return","cancellationUrl"=>"https://localhost:8000/cancel",
        "merchantBusinessLogoUrl"=>"https://localhost:8000/wallet","merchantAccountNumber"=>"HM0409180006","clientReference"=> '1');
      $curl = curl_init();

      curl_setopt_array($curl, array(
        CURLOPT_URL => "https://api.hubtel.com/v2/pos/onlinecheckout/items/initiate",
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_ENCODING => "",
        CURLOPT_MAXREDIRS => 10,
        CURLOPT_TIMEOUT => 30,
        CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
        CURLOPT_CUSTOMREQUEST => "POST",
        CURLOPT_POSTFIELDS => $invoice,
        CURLOPT_HTTPHEADER => array(
          "Authorization: Basic bjJXS0o1Qjo0saMWU4NzA4ZS0wODVkLTQ0NDwUtOTQ3ZS1hZjBiODlkNzI1OWY=",
          "Content-Type: application/json",
          "Postman-Token: 2bd373c8-3669-4f5f-93e9-d3001af0a9cff",
          "cache-control: no-cache"
        ),
      ));