If you see a 404 error when clicking this link[github.com] then it means you have not connected your github and Epic accounts. You need to do that, Epic rule.

Since it's very confusing and troublesome, I've decided to make a dedicated topic for this issue alone. I'm assuming that you have both accounts created (unrealengine[www.unrealengine.com] and github[github.com]) Here we go:

1. Both Github and EpicGames require you to verify your e-mail address. After creating those accounts you have received emails from each (I have tested this all over again now and it takes several minutes unfortunately). You have to wait and do this before continuing.
   https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612725844
   https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612726054
   
2. Go here[www.unrealengine.com] and press the button shown on screen below (it's in a non-default tab, make sure to change it to "Connected Accounts")
   https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612724684
   
3. If you see this screen, it means you've failed at step 1. Go back there.
   https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612727170
   
4. Press the green button here:
   https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612727684
   
5. Log in to github using the credentials that you used when creating an account there
   https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612728270
   
6. Now you should receive an invite on GitHub by Epic. Check your e-mail, should look like this:
   https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612730301
   
7. You will see this page:
   https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612728673
   
8. Press the 'View Invitation' button.
   
9. Click the big green button. You are now officially a member of the private Epic group.
   
10. This is how a properly linked account looks on the Epic's site (with your nickname instead of mine):
    https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1612729386
    
11. These e-mails you receive after all is set up right.
    https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=660780435

Once you do that you just give the synchro app the credentials you used for creating GitHub account. I always go with email and password instead of login and password but both work the same way. Alternatively, use the first link I gave you here[github.com] to download it manually as a *.zip package and paste into the editor folder (that is also explained in more detail in the tutorial).

Once you got that part right, the tutorial should be enough to explain you the rest. Don't be shy to ask if it's not though, I'm just a man and I do make mistakes ;)

Again:

1. The 404 is misleading, but that's how GitHub handles access denied error.
   
2. The sync tool from steam will only work if you give it credentials of the account that is properly connected to unrealengine.com.
   
3. You only need to confirm the invite that you receive on your e-mail and that's it. If you've never used GitHub before, you also need to verify the GitHub e-mail you used so they activate the account fully.

I do see that many people have issues getting this to work, and I really would like to just let you download everything and quit this all nonsense. Sadly I can't. Try checking the e-mail you registered with (if you used different for each site then check both) and confirm that you are you. That's all there is to do.

Should you have any more issues or questions regarding this step, feel free to ask in this topic, I'd like to have it all in one place.