There is a bug with PayPal checkout on early versions of Magento 2. Users may see error
"We can't place the order" after they submit the order on PayPal. This error appears on the order review page where user has to submit order again in order to place it on the website.
Developers report different causes of this issue. In our case it was due to problems with billling address.
If you have Magento 2.1.x and see the following error in your
main.CRITICAL: Exception message: Please check the billing address information. Please enter the street. Please enter the city. Please enter the phone number. Please enter the zip/postal code.
then this fix will help you: https://github.com/Eleanorsoft/m2-fix-paypal-error-on-checkout
This fix disables address validation during PayPal checkout.