You'll need to create an API username for PayPal that's different from your main account username. You can only access PayPal via API only if you have a PayPal Premier / Business / Website Payments Pro account.
Follow these steps:
Either
Click this link
https://www.paypal.com/businessprofile/mytools/apiaccess/firstparty/signature
or
Step 1 - Log in to your PayPal Business account.
Paypal profile access
Step 2 - Click on icon top right with your user name showing.
Step 3 - Select Account settings the Update on API Access
Step 4 - scroll down until you see NVP/SOAP API integration (Classic) now click Manage SDK Credentials
Step 5 - Click Manage API credentials
Step 6 - Ensure the radio button for Request an API signature is active then click Agree and Submit
Step 7 - click Show and make a note of these three values
Show
Step 8 - When prompted enter the username, password and signature into your Agent Capture Assist gateway setup page. If you got stuck at this point during the setup you can log back in to Paytia and you will be taken back to that page.