Invoice Paypal Link Not Working

Hi, I’m unable to find the option in Paypal’s Developer area for “API Username” and “AP Signature” nor live Webhooks information., and get the “Security header is not valid (Error Code: 10002)” error. I realize this error is generally the result of?having miscopied credentials from PayPal, but I can’t seem to find where to get the info for the above.

(Settings > Payment Processors > Paypal Payments Standard > Enabled > Settinsg)

Would appreciate your directing me to the info/documentation page/link and such. Thank you!