Free Software can legally be sold (although I am not stating this is specifically the case of 0 A.D.) See this: https://www.gnu.org/philosophy/selling.en.html The word "Free" in "Free Software" can be misleading due to its ambiguous meaning. GNU and FSF clarify which interpretation should be given to the word "Free". Quoting from https://www.gnu.org/philosophy/free-sw.html.en: [...] think of “free” as in “free speech,” not as in “free beer.” 

@Perzival12soon in R29 there will be https://gitea.wildfiregames.com/0ad/0ad/pulls/8724 with examples, I know it is needed, because @The Undying Nephalim mentioned it many times in his videos about some factions.

So, I just installed the game and every time I boot up the game, I get an error message. The game works fine I just get no sound. I already updated my drivers and tried looking into the issue but can't seem to find anything that helps. I'm also not the most tech savvy so I don't even know where to start on troubleshooting. Any help or advice is appreciated. I included a screenshot of the error message that pops up.

The license states that it is Free software. It legally can’t be sold.

As far as I know, the developers of 0ad themselves have allowed in their legal guidelines that anyone can sell it, as stated in the GPL2 and Creative Commons licenses. As long as the source code is publicly available, there is no legal or ethical impediment to selling 0ad with or without modifications. If they don't want it sold, then they should change the license; using the GNU GPL doesn't necessarily mean that you don't have to pay for the free version. I think people who install 0AD from Steam won't care if it's alpha or beta or has campaigns; as long as they can play a single-player game, that's enough. If you don't do it, someone else will, whether by changing the logo, name, and so on, but keeping the same engine and assets. People don't want perfection; they just want to play. And if the game is open source, then modifying and selling it is completely free and allowed. Richard Stallman must be turning in his grave because Steam prevented a game from being sold under his own license. If 0ad was removed from Steam it was only due to ignorance, but it is completely legal if the correct steps of Steam's guidelines are followed, and if the official studio doesn't do it, someone else will someday.