Same thing happening to me, narrowed it down a little I think:
I’m trying to get it working on an NVIDIA Jetson Nano (running I think nvidia’s custom ubuntu), and followed the steps here to get it installed, which works fine, and the service is getting launched on wifi connection, etc, but I get the same error message as @Sunny when trying to install default plugins.
journalctl doesn’t show any new info when I hit the “unable to connect to jenkins” screen either, but I stumbled on this post while Googling the error, and tried pinging updates.jenkins.io which doesn’t get any responses. As far as I know I’m not behind any proxies etc, any ideas on why the plugin repository might be inaccessible?
(edit: also rebooting didn’t solve anything unfortunately)
(edit 2: this could also be not the issue, because curl -i https://updates.jenkins.io returns 200, and I CAN grab the contents of the webpage with curl? - I can also manually get to the pluginManager endpoint and install stuff from there, so something wrong with the configuration step maybe?)