There are certain errors that can never truly be fixed in live service games like Dota 2 simply because with each new update, something new could go wrong and bring old problems back to the surface.
Dota 2 Error 127, better known as “Game Client Out of Date Error,” is one of the most problematic of those recurring issues.
Typically, this client issue just means you’re trying to load an outdated version of Dota or Steam. So to fix it, you should probably try exiting out of Steam and seeing if the platform needs an update first.
If your version of Steam is up to date, then you should head to your Library and check if Dota 2 has any updates available or that are pending a download. One of these two solutions will typically fix the problem and let you get back to playing the game.
But if neither of these simple update tactics work, you’ll have to get a little more technical.
Before uninstalling the game and seeing if a fresh install will fix things, you should check the game’s properties by right clicking the game in your Library. Once inside the properties menu, scroll down and click on local files and then click “verify integrity of game files…”
Outside of that, you’ll likely need to perform a clean install of either Dota 2 or Steam entirely, unless you want to start clearing Steam’s appcache folder or other system-related fixes.