It looks like your app doesn’t have an IPv4 address. This error can occur when you try to connect over IPv4 (i.e. because your ISP doesn’t support IPv6).
You can tell by going to https://ipv6-test.com/ or trying:
❯ curl -v ipv6.google.com
* Trying [2607:f8b0:4006:80b::200e]:80...
* Immediate connect fail for 2607:f8b0:4006:80b::200e: Network is unreachable
* Failed to connect to ipv6.google.com port 80 after 120 ms: Couldn't connect to server
* Closing connection 0
curl: (7) Failed to connect to ipv6.google.com port 80 after 120 ms: Couldn't connect to server
The solution is to allocate an IPv4 address. This should give you a free shared address:
fly ips allocate-v4 --shared