laravel 7 with CURL - php

Question

need help. im stop with this situation, i got an error that i don't understand. i think because i am weak in this. i have API payment and i use CURL. i actually get the encoded value but when i try using CURL to pas the value to API. i got this setContent() must be of the type string or null.. take a look my code.
$credentials = $this->creds();
$secret_key = $credentials->private_key;
//$secret_key = 'cxl+hwc%97h6+4#lx1au*ut=ml+=!fx85w94iuf*06=rf383xs';
$public_key = $credentials->public_key;
//$public_key = '7)5dmcfy^dp*9bdrcfcm$k-n=p7b!x(t)_f^i8mxl#v_+rno*x';
$merchant_id = "3581";
$dataToHash = "5a8c12eb19016500.00PHPMy Product";
$secure_hash = hash_hmac('sha256', $dataToHash, $secret_key, false);
$auth_hash = hash_hmac('sha256', $public_key, $secret_key, false);
$customer_array = array (
'merchant_id' => $merchant_id,
'merchant_ref_no' => "6a2c14eb19355",
'merchant_additional_data' => "Additional Data",
'amount' => 1500,
'currency' => 'PHP',
'description' => "Description",
'billing_email' => "email#gmail.com",
'billing_first_name' => "jayson",
'billing_last_name' => "claros",
'billing_middle_name' => "curada",
'billing_phone' => "123456789",
'billing_mobile' => "12345678",
'billing_address' => "address1",
'billing_address2' => "address2",
'billing_city' => "None",
'billing_state' => "None",
'billing_zip' => "None",
'billing_country' => "None",
'billing_remark' => "None",
'payment_method' => 'BOG',
'status_notification_url' => 'https://242c3.ngrok.io/callback',
'success_page_url' => "",
'failure_page_url' => "",
'cancel_page_url' => "",
'pending_page_url' => "",
'secure_hash' => $secure_hash,
'auth_hash' => $auth_hash,
'alg' => 'HS256',
);
// $payform_data = json_encode($raw_payform, JSON_UNESCAPED_SLASHES);
// $decoded = utf8_decode(base64_encode(utf8_encode($payform_data)));
$billing_json = json_encode($customer_array, JSON_UNESCAPED_SLASHES);
$datax= base64_encode($billing_json);
$URL = 'https://api.paymentget.com/payform-link?format=json';
$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $URL);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_POSTFIELDS,$datax);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
$rest = curl_exec($curl);
if($err = curl_error($curl)) {
echo $err;
} else {
$decoded = json_decode($rest);
}
return $decoded;
the result should look like this.
eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.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.MHHqWbf74rokfxqdHJ1HXwB8NEGo24uQBjhdMtuRvGc&form_data=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