The 'Unity missing extension of native class' error is typically caused by one of two issues. The first and most common issue is that there's a discrepancy between the Unity engine version and the native plugin version you're trying to use. If the plugin was built with a different version of Unity than the one you are currently using, it might result in this error.
The second potential cause is that the script referencing the native class doesn't have its dependencies properly configured or included. When Unity compiles your scripts, it needs access to all the referenced classes in order to correctly generate the assembly. If Unity cannot locate a referenced class because it's missing or improperly linked, it throws the 'missing extension of native class' error.
Firstly, ensure that your Unity version matches the version required by the native plugin. If they do not match, you may need to upgrade or downgrade your Unity version, or find a version of the plugin compatible with your current Unity version.
Secondly, check the assemblies and dependencies in your project. Make sure that the scripts referencing the native class have correctly imported and linked to the necessary libraries or files. If you're using a custom native plugin, verify that it's placed in the "Assets/Plugins" directory and it has been properly set up according to the platform (iOS/Android) you're compiling for.
Additionally, if you're working with iOS and using Objective-C, be mindful that file extensions matter. *.mm files are treated as Objective-C++ and can call both Objective-C and C++ libraries, whereas *.m files are treated as plain Objective-C.
Finally, try cleaning and rebuilding your Unity project. Sometimes, stale or corrupted build data can cause issues, and a clean rebuild may resolve them.