I am trying to set up a PayPal form

When I test it, I get invalid credentials even though I copied the API data directly from Paypal.

I see that you are experiencing issues with setting up API. Providing exact error message would help us in assisting you quickly. Generally the issue occurs when only the API key or signature is wrong. I request you to go through the document provided below on obtaining API credentials.

How-to-Obtain-PayPal-API-Credentials

Please make sure that you are not having trailing or leading spaces while you paste the key into the form. You may also try regenerating the keys only if you are not using these keys elsewhere.

Also, I see that you are trying using PayPal Pro. Please take a look at the following document if you are not sure which to be used.

Difference-Between-PayPal-PayPal-Pro

Request you to retry again and let us know if the issue still persists. We are happy to assist you further.