I recently ran into this bug after updating IntelliJ, and while richbai90's solution did fix the immediate issue, it also broke compiling the mod into a jar (the assets folder gets included twice). After some digging around, I eventually found the root of the issue: IntelliJ was delegating the build task to Gradle, which put the assets and classes in separate folders, and Forge didn't know they belong to the same mod. The solution that worked for me was to build and run using the IDE, which is in the Settings dialog under Build, Execution, Deployment | Build Tools | Gradle (the help page has more detailed instructions). On older versions of IntelliJ, this was called "Delegate IDE build/run actions to gradle" (see the help page).