It depends on the software, just by being open source doesn't mean you give up all rights to earn money.

One example is the Aseprite editor https://github.com/aseprite/aseprite/ it is open source and you can build it from source and use it fine, but most average people can't be bothered to build from source, and the prebuilt binary downloads are paid, many people pay for this software.

Another example is apps that may run on the cloud, you may open source it so people can self-host it, but you'd still have a centrally hosted version that's paid.

It all depends on what type of software you are trying to sell.