Port 80 or 443 is in use

rexenv names what is holding a port rather than reporting a generic failure to start. From a terminal:

rex doctor

Find the holder yourself

sudo lsof -nP -iTCP:443 -sTCP:LISTEN
sudo lsof -nP -iTCP:80  -sTCP:LISTEN

sudo matters here: without it you will not see processes owned by root, and the thing holding 443 is usually owned by root.

The usual suspects

What	How to stop it
Homebrew nginx	sudo brew services stop nginx
Homebrew httpd	sudo brew services stop httpd
Laravel Valet	valet stop
Herd	Quit it from the menu bar
Docker publishing 80/443	Stop the container or remap its ports
macOS Web Sharing	System Settings → Sharing

A shadow bind can hide the real holder

Some tools bind 127.0.0.1:443 rather than 0.0.0.0:443. A second process can then bind 0.0.0.0:443 with no error — and requests to localhost still go to the first one.

So lsof showing rexenv on the port is not proof rexenv is the one answering. Check who answers:

curl -sI https://myshop.rex | head -1

If that fails while rexenv insists it is serving, something is shadow-bound in front of it. Stopping the other tool completely is the fix; stopping “just its site” is not.

“Nothing is on :443”

A different problem with the same-looking symptom: nothing is holding the port, rexenv just is not serving yet. Start all in the app, or:

rex start

The two cases are told apart on purpose, because sending someone hunting for a program that is not running is worse than saying nothing.

After stopping the other tool

rex restart

The edge re-binds on start. It does not poll for the port becoming free.