Getting a "Merchant Login ID or Password is invalid" error when submitting form

"merchant login ID or password is invalid or the account is inactive"

I believe the issue has something to do with your Authorize.Net credentials. You can find more information about it here: https://support.authorize.net/s/article/Error-13-The-merchant-login-ID-or-password-is-invalid-or-the-account-is-inactive

Try to update your Authorize.Net credentials or make sure that the credentials you entered in the integration settings of the payment field in your form are correct as this normally solves the issue you are having.