Oops! Looks like something went wrong - service unavailable.
This is most likely a temporary error. Please wait a bit and try again later.
If the problem persists after several attempts, it may also be because you don't have enough funds or are missing a valid funding source on your PayPal account.
It could also be that you are in a paypal zero country, if so, you will have to contact paypal support instead.
Note that it's possible the transaction went through even though you got an error message, please double check your PayPal account balance.
You can also refer to the PayPal help center here.