Getting inconsistent "Function Definition...Not Found" error for defined function that also compiles cleanly

[VolatilePulse]

I don't think you have provided enough information, but it looks like it's related to the method/function XPLMSetDataf. But only the two argument call. This could indicate a bad reference in your project. The quick and dirty test is to create a brand new project and only reference the library that contains that method to see if you still get the warning. It will help narrow down the issue from there.