There is no offline mode.
MainWP has 2 plugins, one on the controller and one on the remote site. They interact to notify of updates. I run the MainWP controller in Local. There are some intricacies of setting up MainWP, I think solutions are in their forums. I’ve searched there. You will need to identify you local copy of OpenSSL.
I got the above error and it didn’t seem to effect function. I have dug into it yet to see why this is happening. I have changed laptops so this might be the issue. Even on Mac I had to do a manual setup like I was on Windows.