cURL request causing form validation error

I'm working on a cURL request when a user submits an existing form to send some data to an outside service.

I've added this function with this line: $form['actions'][$action]['#submit'][] = 'my_module_push_data';

And my cURL requests:

    $ch = curl_init();
    // Get access token here
    curl_setopt($ch, CURLOPT_URL, 'https://resturl.com&id=' . $id . '&secret=' . $secret);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    $result = curl_exec($ch);
    curl_close($ch);
    $result = json_decode($result, true);

    // Then submit data
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, 'https://resturl.com/submit.json?access_token=' . $result['access_token']);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($formData));
    curl_setopt( $ch, CURLOPT_HTTPHEADER, array('Content-Type:application/json'));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
    $server_output = curl_exec($ch); // <--- This line causes issues
    $server_output = json_decode($server_output);

With this code, I'm getting this error:

Uncaught PHP Exception LogicException: "Form errors cannot be set after form validation has finished." at /mnt/gfs/.../.../docroot/core/lib/Drupal/Core/Form/FormState.php

I've gone line by line through this cURL request and the line I marked (second curl_exec() causes this error. If I omit that line it works fine. The first curl_exec() works fine and I verified it's returning an access token.

What am I doing wrong here? Thank you!

EDIT: Full function (obfuscated some details)

/**
 * Custom submit handler to push data into Service
 */
function my_module_service_signup(array $form, \Drupal\Core\Form\FormStateInterface $form_state) {
    $vals = $form_state->getValues();
    $formData = [];
    $myVals = [];

    $clientId = Settings::get('service_api_id');
    $clientSecret = Settings::get('service_api_secret');
    $accountId = 'abc123456';
    $formData['unique_id'] = '1234';

    $myVals = [
        'company' => $vals['field_company'][0]['value'],
        'firstName' => $vals['field_first_name'][0]['value'],
        'lastName' => $vals['field_last_name'][0]['value'],
        ...
    ];

    $formData['input'][0]['fields'] = $myVals;

    try {
        $getClient = \Drupal::httpClient();
        $request = $getClient->post('https://' . $accountId . '.resturl.com/get/token?my_id=' . $clientId . '&my_secret=' . $clientSecret);
        $response = json_decode($request->getBody());
    } catch(RequestException $e) {
    // The LogicException is not caught here
    }
}