This error can appear for a variety of reasons. We will go through the most common reasons and solutions.

Your antivirus is blocking WeMod

Some antiviruses will stop WeMod when it attempts to inject into a running process(the games). To resolve this issue, follow the directions Here to add WeMod to your antivirus's whitelist.

Your game version is incompatible

A trainer may not be compatible with your version of the game. The trainer could be out of date, your game could be out of date, or your game is from a different DRM. Make sure your game is listed as compatible in WeMod. If it is listed as not available and shows a message stating that it needs to be tested or updated, please check the game queue to see its current position. Clicking "notify me" will alert you when the trainer is updated. 

The game requires extra steps to disable anti-tamper

Occasionally a game will have anti-tamper in singleplayer that requires file patching or extra steps when launching the game. Please check the trainer notes for special launch instructions.

Please create a new post on our community forums if this doesn't resolve your issue, one of our knowledgeable support staff will be able to provide you with more details and assist you with resolving the issue.