I can’t remember if turning on OpenGL driver was my issue or not with this…
As long as your config files is correct (it isnt a hard config file I will post mine below)…
Try opening a terminal window and type sudo raspi-config
go to “Advanced Options” and then “OpenGL Driver”

Depending on the hardware you have or what PI model you are using you may have just one option or a couple…
IF you have multiple options like I have seen … one was FAKE KMS and the other was FULL KMS…
I had to choose FULL KMS (i think for raspberry pi3)… on my raspberry pi 4 I believe there is only one option…choose it.

(might want to reboot)… cant remember if needed or makes you.

Then try again … make sure your device is on the same wifi network as your mirror …
You should see your mirror name …

I can’t think of what more there could be … unless your router is blocking the default port?
You did do npm install in the module directory after cloning it right?

My config for the module…

{ module: 'MMM-Screencast', position: 'center', // This position is for a hidden <div /> and not the screencast window config: { position: 'center', height: 300, width: 500, } },