Please post code as code, not as screenshots. Same with error messages as text. If we can’t get a complete picture of the code it’s very hard to say what’s going on. All that error says is that you’re trying to use the index_html variable which hasn’t been previously declared. If you have shared variables (and functions) between main.cpp and WebServer.cpp, then create a header file (usually one for each .cpp file) where you declare these, then include them in the .cpp file. This is detailed in multiple topics in this forum, see e.g. Tutorial for creating multi cpp file arduino project - #36 by NormanDunbar.