Hi @Elydreams
It sounds like you might have your checkout in "Sandbox" mode rather than production. When you open your checkout through your PayPal Checkout button, does the URL show paypal.com or sandbox.paypal.com?
If it shows sandbox.paypal.com, you'll want to get your Live Client ID from your Developer account on developer.paypal.com. Log in with your Live username and password, then, select the "Live" toggle next to the "Sandbox" toggle. Once you're viewing the "Live" apps, either create a new app or select an existing app. Open the app and use those credentials on your site to set up your checkout. This documentation can also help with where you might have to add those credentials.
I hope that helps!
-Aaron
