Hi,
It could be that the credentials you provided don't match with how you configured the "sandbox" setting in the payment method.
A live PayPal account can only be used with the sandbox setting turned off and a with the setting turned on, you need to use a PayPal sandbox account.
But it could also be a javascript error from something else preventing the PayPal javascript from displaying the payment buttons below that text.
So first thing is to check that sandbox account.
Then, if that is correct, on the last page with that text, open the console of your browser and look at the javascript error messages. they should indicate where the problem is. If you can provide what you have in the console, we should be able to tell you what to do.