TLDR: If on Windows and lacking Admin status within your system, try giving the user account admin status and then clicking the “fix unassociated files“ button in the launcher.
I had this issue for a long time. I’m talking years. I tried to address it many times. I would click the “fix” button to remedy the issue when opening the launcher, it would appear to work only for nothing to change. I attempted to manually address the lack of association following the advice from multiple threads and forum posts. (This one included, I’ll post a couple below) The issue persisted across fresh installs and even new devices. Nothing seemed to work. I eventually gave up and got around it using the launcher. This was on a work device, and I couldn’t justify spending more time on something that I could get around with just a few extra clicks. I eventually came across a bug that all but required me to fix the problem at its root instead of finding workarounds. (I needed to generate new Visual Studio files) So I reached out to our IT department. They also tried several times to address the lack of association with reg-edit and attempted a few things from the terminal that didn’t change much. The thing that finally did it was granting my user account administrator status and clicking the “fix” button that appeared when opening the launcher. (We went back and then removed the admin status after.) They had been giving me a credential to troubleshoot, but something about the relationship between user settings and the fix unreal applies to the problem required my account to be an admin for it to actually take effect. Our conclusion was that whenever I was “running as administrator,” the changes would affect the admin user I had credentials for, not my user account. So the change would appear to take place, but not actually affect anything. I hope this might help anyone in a similar situation.
Here are other places where people dealt with a similar problem:
https://www.reddit.com/r/unrealengine/comments/13gn4bu/i_cant_open_uproject_files/