First time trying to use Pinegrow

I have bought the one-time licence version of Pinegrow after having looked at the demo (no problems running that). I am running on Linux (Debian). Now, with the full activated version, I am getting an error message:
The internal webserver is not accessible at undefined. Error received: TypeError: Failed to execute 'fetch' on 'Window': Failed to parse URL from http://:40000/pginternal/detect

I have done various searches for solutions and also tried to open a port in the firewall, as suggested, but no joy. On the Pinegrow website it says to try pointing my browser to and “You should get a short 404 - Not found message if the web server is accessible. If that’s the case contact us about the issue.”, which is the case.

I will contact support, but also asking on the forums for any possible help.

Many thanks.

apologies, I seem to have sorted it out very quickly!
I simply typed ‘hostname’ in a terminal and then entered the result in the Pinegrow settings (it was blank). This seems to have sorted it out.