@amteza After long debugging, I finally got fping and flent working on the Macbook. I searched through Flent github issues and came across https://github.com/tohojo/flent/issues/240#issuecomment-931256026 . There's no fping6 command in macOS so I instead tried:
% fping -D -c 1 localhost
[1657503443.62438] localhost : [0], timed out (NaN avg, 100% loss)
localhost : xmt/rcv/%loss = 1/0/100%
Turns out macOS firewall has a Stealth Mode (System Preferences -> Security & Privacy -> Firewall -> Firewall Options... -> Enable Stealth Mode). That option was enabled. After disabling "Enable Stealth Mode" I confirmed fping to localhost is working:
% fping -D -c 1 localhost
[1657503635.17864] localhost : [0], 64 bytes, 0.116 ms (0.116 avg, 0% loss)
localhost : xmt/rcv/%loss = 1/1/0%, min/avg/max = 0.116/0.116/0.116
After that change, flent works.