Is this something Amazon have put in place?
TheGreatOne
Amazon (and other providers) are constantly expanding their
VPN detection capabilities and blocking known VPN servers ..
.. they are even obliged to do so for licensing reasons.
Since I didn't use Express VPN that year, I don't have any experience with it.
With NordVPN + SurfShark, almost all US servers work for me for amazon.com
In the UK, VPN usage has recently increased significantly
in order to access p*rn sites without strict age verification.
This may be forcing the VPN blocks, or you may have just
happened to connect to a server that is known/blocked.
So just disconnect and reconnect if manual server selection is not possible.