I have the same issue on Macos Big Sur.
```shell
fping -v
fping: Version 5.0
```

```shell
flent tcp_download -H netperf-eu.bufferbloat.net -v
Starting Flent 2.0.1 using Python 3.9.6.
Executing test environment file /usr/local/lib/python3.9/site-packages/flent/tests/tcp_download.conf
Looking up hostname 'netperf-eu.bufferbloat.net'.
Executing test environment file /usr/local/lib/python3.9/site-packages/flent/tests/tcp_download.conf
Gathering local metadata
Executing 'uname -s' on localhost
Executing 'uname -r' on localhost
Executing 'find /sys/module -name .note.gnu.build-id' on localhost
Executing 'sysctl -e net.core.rmem_max net.core.wmem_max net.ipv4.tcp_autocorking net.ipv4.tcp_early_retrans net.ipv4.tcp_ecn net.ipv4.tcp_pacing_ca_ratio net.ipv4.tcp_pacing_ss_ratio net.ipv4.tcp_dsack net.ipv4.tcp_fack net.ipv4.tcp_sack net.ipv4.tcp_fastopen net.ipv4.tcp_syncookies net.ipv4.tcp_window_scaling net.ipv4.tcp_notsent_lowat net.ipv4.tcp_limit_output_bytes net.ipv4.tcp_timestamps net.ipv4.tcp_congestion_control net.ipv4.tcp_allowed_congestion_control net.ipv4.tcp_available_congestion_control net.ipv4.tcp_mem net.ipv4.tcp_rmem net.ipv4.tcp_wmem net.ipv4.tcp_moderate_rcvbuf net.ipv4.tcp_no_metrics_save' on localhost
Looking up hostname 'netperf-eu.bufferbloat.net'.
Executing 'ip route get 193.10.227.30' on localhost
Executing 'route -n get 193.10.227.30' on localhost
Executing 'tc qdisc show dev en0' on localhost
Executing 'tc -s qdisc show dev en0' on localhost
Executing 'tc class show dev en0' on localhost
Executing 'ethtool -k en0' on localhost
Executing 'for i in /sys/class/net/en0/queues/tx-*; do [ -d $i/byte_queue_limits ] && echo -n "$(basename $i) " && cat $i/byte_queue_limits/limit_max; done' on localhost
Executing 'basename $(readlink /sys/class/net/en0/device/driver)' on localhost
Executing 'ip link show dev en0' on localhost
Executing 'ifconfig en0' on localhost
Executing 'ethtool en0' on localhost
Starting tcp_download test. Expected run time: 70 seconds.
which: /Users/stanislas/.n/bin/netperf is not an executable file
which: /Applications/Visual Studio Code.app/Contents/Resources/app/bin/netperf is not an executable file
which: /usr/local/opt/openssl@1.1/bin/netperf is not an executable file
which: /usr/local/opt/icu4c/sbin/netperf is not an executable file
which: /usr/local/opt/icu4c/bin/netperf is not an executable file
which: /usr/local/opt/mysql-client/bin/netperf is not an executable file
which: /usr/local/opt/gettext/bin/netperf is not an executable file
which: /users/stanislas/bin/netperf is not an executable file
which: Found netperf executable at /usr/local/bin/netperf
which: /Users/stanislas/.n/bin/fping is not an executable file
which: /Applications/Visual Studio Code.app/Contents/Resources/app/bin/fping is not an executable file
which: /usr/local/opt/openssl@1.1/bin/fping is not an executable file
which: /usr/local/opt/icu4c/sbin/fping is not an executable file
which: /usr/local/opt/icu4c/bin/fping is not an executable file
which: /usr/local/opt/mysql-client/bin/fping is not an executable file
which: /usr/local/opt/gettext/bin/fping is not an executable file
which: /users/stanislas/bin/fping is not an executable file
which: Found fping executable at /usr/local/bin/fping
which: /Users/stanislas/.n/bin/ping is not an executable file
which: /Applications/Visual Studio Code.app/Contents/Resources/app/bin/ping is not an executable file
which: /usr/local/opt/openssl@1.1/bin/ping is not an executable file
which: /usr/local/opt/icu4c/sbin/ping is not an executable file
which: /usr/local/opt/icu4c/bin/ping is not an executable file
which: /usr/local/opt/mysql-client/bin/ping is not an executable file
which: /usr/local/opt/gettext/bin/ping is not an executable file
which: /users/stanislas/bin/ping is not an executable file
which: /usr/local/bin/ping is not an executable file
which: /usr/local/sbin/ping is not an executable file
which: /usr/bin/ping is not an executable file
which: /bin/ping is not an executable file
which: /usr/sbin/ping is not an executable file
which: Found ping executable at /sbin/ping
WARNING: Found fping, but it outputs broken timestamps (off by 1632696006.860897s). Not using.
ERROR: Runner Ping (ms) ICMP failed check: Cannot parse output of the system ping binary (/sbin/ping). Please install fping v3.5+.
```

I think the warning about `fping`'s broken timestamp might be the issue.

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