"Access Denied" errors when trying to check out or pay usually indicate some sort of network error.
If you are using a VPN, try disabling it.
If you are using a custom firewall, that may be causing problems.
Try paying using your phone or laptop when not connected to your usual network wifi.
See here for further suggestions of things to try:
https://community.ebay.com/t5/Technical-Issues/Access-Denied/m-p/29845776#M56948
