The error appears for five main reasons. Diagnosing which one applies to you will save hours of trial and error.

I've organized the following solutions from the to the more technical approaches. Start with Solution 1 and work your way down until the issue is resolved.

This removes any stubborn mod files or permission errors that Steam verification might miss.

This is the for Steam users and has a documented success rate of around 75%. It works by having Steam check every file in your game installation against the official versions and automatically replace any that are missing or corrupted.

Your antivirus software may have mistakenly flagged and quarantined the file.