I had this problem a week or two ago and I posted here but nothing from the suggested solutions seemed to work. I have managed to get some more details on it, in case anyone can help me, because this issue is causing me huge productivity problems and I really want it solved.
So I'm using Linux mint distribution, and I've installed Lyx via the software manager. Every time that I open a new document, I get an error claiming that some files are missing, and until they are installed, compiling the files or exporting them as pdfs is not possible. The file in question is called "extarticle.cls". Additionally, under Document->Settings->Document Class, every document class is marked as unavailable.
Here is the thing though.. this file is not missing... I have located it inside a folder associated with Lyx. It's definitely not missing.
I really need help with this one guys... It's been weeks since the problem came up and I really just want to be rid of it.. Why does this thing not work? Any help?