Assuming that you have a verified PayPal account enough money i your PayPal account balance or in a credit/debit card linked with your PayPal account, you can try these (one step at a time and try to make the purchase again):
1. Try again a few minutes later (after 30 minutes in example).
2. Clear the cache and cookies of your browser (ctrl+shift+delete) and reboot your router modem.
3. Try with a different browser.
4. Try with a different internet connection (if it's possible).
If all the above fail, then you need to contact directly PayPal's support and ask for assistance