Lots of red lines.

Is this inside of the cpp file or the header file?

You seem to be mixing definitions with implementations.

Seeing that you are including the SceneAssetAction.h at the top I’m 90% sure it’s the cpp file of SceneAssetAction.

Is scene asset in the same locations as your current file? Please declare what the file is called.

OpenAssetEditor and GetCategories() && GetName() are followed by overrides. This should be stated in the header file and not the cpp file

in the cpp file you need their implementation

FText SceneAssetAction::GetName() {
// (return  your rest of code)
}
void SceneAssetAction::OpenAssetEditor( /// passed in parameters ) {
// your code here
}

same for the GetCategories.

in the cpp file it should be

SceneAccetAction::GetCategories(){
return _assetCategory;
}  

or you could do an inline implementation directly in the header if the function is small

In the implementation omit the override statement (it’s header only)

I would take a long hard look at the tutorial again slowly and double check the separation between header and cpp file.