If you use v0.4.0, inclease the value of HEART_BEAT_TIMEOUT to 120.
You can change the value on your Nerves project directory, edit rel/vm.args.eex like following.
## Enable heartbeat monitoring of the Erlang runtime system
-heart -env HEART_BEAT_TIMEOUT 180
This is because it takes time to generate the entropy, by rngd, needed to run the Erlang VM.
Especially after burned SD first boot takes long time, cause it includes initialized /root, /data/, partition.
pojiro
commented
2 years ago 
If you use v0.4.0, inclease the value of HEART_BEAT_TIMEOUT to 120.
You can change the value on your Nerves project directory, edit
rel/vm.args.eexlike following.This is because it takes time to generate the entropy, by
rngd, needed to run the Erlang VM. Especially after burned SD first boot takes long time, cause it includes initialized /root, /data/, partition.