Paypal Error 500

I’ve already inserted all my info correctly so could you tell me what’s up? Take a look at it: 

I’ve tested it in sandbox mode and it worked, but the live version doesn’t. I’ve read the other posts and it’s said that error 500 comes from server side and that the credentials are incorrect. Please do get back to me on this matter as it’s an important part of a funding website. Thanks. 

Regards,

Ben