Ensure you have run this version at least once in your Minecraft Launcher.
If your version did not come with OptiFine bundled, download the 1.12.2 version of OptiFine and place it in the .minecraft/libraries or mods folder created by Wurst. Safety Warning Download Wurst v6.35 MC1.12 OF
To download (with OptiFine compatibility), you should use the official Wurst Client website to ensure you are getting a safe and authentic version. Official Download Link Ensure you have run this version at least