That error is saying that retroshare is not in the repositories, so I looked it up and it looks like you need to add the package manually, which they have instructions to do here. If you want, I can elaborate later but I'm on mobile rn.
you should maybe try the software RetroShare
it's an encrypted peer to peer software where you can chat with friends, share folders, have some forums and other stuff the software is pretty cool
Some good choices:
You don't need a lot of money to have a website.
Additionally, by its design, the Internet is very difficult to take down. Sure, having your own server costs money, but you can control everything on it. The problem Parler had was having a traditional website. The server is the weak point. Decentralized forums are near the point of production.
Here is an "instruction manual to decentralized movements" (a github project). It contains much more than what you ask for but may offer elements of reply. I think it does a good job of outlining the role forums have to play.
Retroshare is a peer-to-peer solution that I have no personal experience with but might seem promising.
The Medium article about digital resistance mentioned by u/NoMordacAllowed is also full of resources.
https://retroshare.cc/
https://en.wikipedia.org/wiki/Retroshare
For large 7-Zip or WinRAR password protected file transfers that uses a direct link try Swiss Transfer. Chat on something else, copy/paste the link.
Some communities have found success with sharing collections over Retroshare. It's a lot like DC++, but modernized and decentralized and you can run it over tor. Here is my certificate.