Content Security Policy

If your checkout page is using the Content-Security-Policy header, you'll need to add a few things to your whitelist.

PolicyOriginNotes
script-srcassets.primer.iorequired
frame-srcassets.primer.iorequired
style-src'unsafe-inline'required
connect-srcapi.sandbox.primer.iorequired
frame-src*.cardinalcommerce.com3DS
frame-srcc.paypal.comFor PayPal Payments
child-srcc.paypal.comFor PayPal Payments
script-srcc.paypal.comFor PayPal Payments