I got the same error you got and worked around it, don’t know if it’s the same problem though, but here’s what I know…
I just set up a local Mattermost cribbed using the Docker compose file which is published in mattermost-app-examples, and used that to launch Mattermost on a local machine. I tweaked the docker compose to deploy 9.3. I installed the apps plugin and then tried to install focalboard, got the same error you got, NetworkError when attempting to fetch resource.
The Docker compose file has a setting…
MM_FILESETTINGS_MAXFILESIZE: 123524266
I increased this number (added a zero on the end), and the install worked.
Not obvious to me why that setting affected the install, but I just got to the point of hacking around with the settings. The ‘default’ setting in the docker compose file is noticeably a smaller number than the size of the focalboard plugin file.
Good luck, I hope you fix your problem.