PayPal sandbox- Transaction Invalid

Hello

I am battling with the PayPal setup and sandbox test.
PayPal has approved my PayPal application. I used your theme a couple years ago and am using the same details as then.

I have entered the sandbox PayPal details in the backend per instructions. When i create a project to test:
1. It is forcing me to choose a reward even though i have disabled rewards
2. If i enable rewards i commit to funding and receive the following message “This transaction is invalid. Please return to the recipient’s website and try again.”

Please could you assist/check that my details are correct?